block: Move op_blocker check from block_job_create to its caller

It makes no sense to check for "any" blocker on bs, we are here only
because of the mechanical conversion from in_use to op_blockers. Remove
it now, and let the callers check specific operation types. Backup and
mirror already have it, add checker to stream and commit.

Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Benoit Canet <benoit@irqsave.net>
Reviewed-by: Jeff Cody <jcody@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
diff --git a/blockjob.c b/blockjob.c
index 60e72f5..7d84ca1 100644
--- a/blockjob.c
+++ b/blockjob.c
@@ -41,7 +41,7 @@
 {
     BlockJob *job;
 
-    if (bs->job || !bdrv_op_blocker_is_empty(bs)) {
+    if (bs->job) {
         error_set(errp, QERR_DEVICE_IN_USE, bdrv_get_device_name(bs));
         return NULL;
     }