fs/lock: add helper locks_owner_has_blockers to check for blockers

Add helper locks_owner_has_blockers to check if there is any blockers
for a given lockowner.

Reviewed-by: J. Bruce Fields <bfields@fieldses.org>
Signed-off-by: Dai Ngo <dai.ngo@oracle.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>