Allow biodone() to be used as a completion routine.

An ordered series of BIO_READ and BIO_WRITE operations are
typically done as:

	while (work to do) {
		setup bp for I/O
		g_io_request(bp, consumer);
		biowait(bp);
	}

Here you need to have biodone() called at the completion of
the I/O to set the BIO_DONE flag and awaken the biowait(). The
obvious way to do this would be to set bio_done = biodone, but
biodone() will only take the desired action if bio_done == NULL.
The relevant code at the end of biodone() is:

	done = bp->bio_done;
	if (done == NULL) {
		mtxp = mtx_pool_find(mtxpool_sleep, bp);
		mtx_lock(mtxp);
		bp->bio_flags |= BIO_DONE;
		wakeup(bp);
		mtx_unlock(mtxp);
	} else
		done(bp);

This code would infinitely recurse if biodone() is specified as the
routine to use at completion. So before this change, a wrapper done
function had to be written:

static void
g_io_done(struct bio *bp)
{

	bp->bio_done = NULL;
	biodone(bp);
	bp->bio_done = g_io_done;
}

This commit changes

	if (done == NULL)

to

	if (done == NULL || done == biodone)

which eliminates the need for the wrapper function.

Reviewed by:  kib
Sponsored by: Netflix

sys/kern/vfs_bio.c

@@ -4384,7 +4384,11 @@ biodone(struct bio *bp)
 		atomic_add_int(&inflight_transient_maps, -1);
 	}
 	done = bp->bio_done;
-	if (done == NULL) {
+	/*
+	 * The check for done == biodone is to allow biodone to be
+	 * used as a bio_done routine.
+	 */
+	if (done == NULL || done == biodone) {
 		mtxp = mtx_pool_find(mtxpool_sleep, bp);
 		mtx_lock(mtxp);
 		bp->bio_flags |= BIO_DONE;