lib/blob: force execution of queued persists

When performing snapshot creation the I/O is frozen
during the process. The blob persists for extent page
allocation is delayed until snapshot creation is finished.

This results in multiple blob persists executing one after
the other, with only intent of writing out updated extent table
pointing to new extent pages.
Since blob->state is marked DIRTY before issuing each persist,
but a single persist completion marks state CLEAR.

Blob serialize correctly expects each persist to contain
dirtied metadata, in order to avoid unnecessary md writes.
Since all other instances of marking blob DIRTY is explicit,
assert in blob serialize is left as is.

Instead when running the queued up blob persists, the blob
state is marked DIRTY.

Side effect is that it will write out same md in some cases.

Fixes spdk#1909

Signed-off-by: Tomasz Zawadzki <tomasz.zawadzki@intel.com>
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/7640 (master)

(cherry picked from commit 5093518)
Change-Id: I39f37299f3f0ebfccbdd4063781b5ecce286e993
Signed-off-by: Krzysztof Karas <krzysztof.karas@intel.com>
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/7677
Reviewed-by: Tomasz Zawadzki <tomasz.zawadzki@intel.com>
Reviewed-by: Aleksey Marchuk <alexeymar@mellanox.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>

lib/blob/blobstore.c

@@ -1642,6 +1642,7 @@ blob_persist_complete(spdk_bs_sequence_t *seq, struct spdk_blob_persist_ctx *ctx
 	free(ctx);
 
 	if (next_persist != NULL) {
+		blob->state = SPDK_BLOB_STATE_DIRTY;
 		blob_persist_check_dirty(next_persist);
 	}
 }