xfs_scrub: fix queue-and-stash of non-contiguous verify requests

xfs_scrub: fix queue-and-stash of non-contiguous verify requests

read_verify_schedule_io is supposed to have the ability to decide that a
retained aggregate extent verification request is not sufficiently
contiguous with the request that is being scheduled, and therefore it
needs to queue the retained request and use the new request to start
building a new aggregate request.

Unfortunately, it stupidly returns after queueing the IO, so we lose the
incoming request.  Fix the code so we only do that if there's a run time
error.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>