Stop ganging due to past vdev write errors

= Problem

While examining a customer's system we noticed unreasonable space
usage from a few snapshots due to gang blocks. Under some further
analysis we discovered that the pool would create gang blocks because
all its disks had non-zero write error counts and they'd be skipped
for normal metaslab allocations due to the following if-clause in
`metaslab_alloc_dva()`:
```
	/*
	 * Avoid writing single-copy data to a failing,
	 * non-redundant vdev, unless we've already tried all
	 * other vdevs.
	 */
	if ((vd->vdev_stat.vs_write_errors > 0 ||
	    vd->vdev_state < VDEV_STATE_HEALTHY) &&
	    d == 0 && !try_hard && vd->vdev_children == 0) {
		metaslab_trace_add(zal, mg, NULL, psize, d,
		    TRACE_VDEV_ERROR, allocator);
		goto next;
	}
```

= Proposed Solution

Get rid of the predicate in the if-clause that checks the past
write errors of the selected vdev. We still try to allocate from
HEALTHY vdevs anyway by checking vdev_state so the past write
errors doesn't seem to help us (quite the opposite - it can cause
issues in long-lived pools like the one from our customer).

= Testing

I first created a pool with 3 vdevs:
```
$ zpool list -v volpool
NAME        SIZE  ALLOC   FREE
volpool    22.5G   117M  22.4G
  xvdb     7.99G  40.2M  7.46G
  xvdc     7.99G  39.1M  7.46G
  xvdd     7.99G  37.8M  7.46G
```

And used `zinject` like so with each one of them:
```
$ sudo zinject -d xvdb -e io -T write -f 0.1 volpool
```

And got the vdevs to the following state:
```
$ zpool status volpool
  pool: volpool
 state: ONLINE
status: One or more devices has experienced an unrecoverable error.
...<cropped>..
action: Determine if the device needs to be replaced, and clear the
...<cropped>..
config:

	NAME        STATE     READ WRITE CKSUM
	volpool     ONLINE       0     0     0
	  xvdb      ONLINE       0     1     0
	  xvdc      ONLINE       0     1     0
	  xvdd      ONLINE       0     4     0

```

I also double-checked their write error counters with sdb:
```
sdb> spa volpool | vdev | member vdev_stat.vs_write_errors
(uint64_t)0  # <---- this is the root vdev
(uint64_t)2
(uint64_t)1
(uint64_t)1
```

Then I checked that I the problem was reproduced in my VM as I the
gang count was growing in zdb as I was writting more data:
```
$ sudo zdb volpool | grep gang
        ganged count:              1384

$ sudo zdb volpool | grep gang
        ganged count:              1393

$ sudo zdb volpool | grep gang
        ganged count:              1402

$ sudo zdb volpool | grep gang
        ganged count:              1414
```

Then I updated my bits with this patch and the gang count stayed the
same.

Reviewed-by: Mark Maybee <mark.maybee@delphix.com>
Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Signed-off-by: Serapheim Dimitropoulos <serapheim@delphix.com>
Closes #14003

module/zfs/metaslab.c

@@ -5201,12 +5201,11 @@ metaslab_alloc_dva(spa_t *spa, metaslab_class_t *mc, uint64_t psize,
 		ASSERT(mg->mg_initialized);
 
 		/*
-		 * Avoid writing single-copy data to a failing,
+		 * Avoid writing single-copy data to an unhealthy,
 		 * non-redundant vdev, unless we've already tried all
 		 * other vdevs.
 		 */
-		if ((vd->vdev_stat.vs_write_errors > 0 ||
-		    vd->vdev_state < VDEV_STATE_HEALTHY) &&
+		if (vd->vdev_state < VDEV_STATE_HEALTHY &&
 		    d == 0 && !try_hard && vd->vdev_children == 0) {
 			metaslab_trace_add(zal, mg, NULL, psize, d,
 			    TRACE_VDEV_ERROR, allocator);