blobfs: Add trace info for request allocation.

Our blobfs allocates fixed number of reqs on three different channels
(the default size is 512), if the req cannot be allocated, our blobfs
related function will do nothing.

With this debug info added, we can easily know the issues.
If the fixed number of requests are not enough, we need to change the
design, using malloc/free together with the fixed number of requests.

During high presure test, I think that if the reqs are not allocated,
it will cause the sync, flush related issues.

Change-Id: Ied8cd5197c4f87b861bd7dc09edd5a095fdcc802
Signed-off-by: Ziye Yang <ziye.yang@intel.com>
Reviewed-on: https://review.gerrithub.io/c/spdk/spdk/+/448596
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>

lib/blobfs/blobfs.c

@@ -273,6 +273,7 @@ alloc_fs_request(struct spdk_fs_channel *channel)
 	}
 
 	if (req == NULL) {
+		SPDK_ERRLOG("Cannot allocate req on spdk_fs_channel =%p\n", channel);
 		return NULL;
 	}
 	memset(req, 0, sizeof(*req));