xfs: fix some minor sparse warnings

A couple of simple locking annotations and 0 vs NULL warnings.
Nothing that changes any code behaviour, just removes build noise.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Ben Myers <bpm@sgi.com>

diff --git a/fs/xfs/xfs_log.c b/fs/xfs/xfs_log.c
index 5372d58..a2dea108 100644
--- a/fs/xfs/xfs_log.c
+++ b/fs/xfs/xfs_log.c
@@ -257,7 +257,8 @@
 	struct xlog		*log,
 	struct xlog_grant_head	*head,
 	struct xlog_ticket	*tic,
-	int			need_bytes)
+	int			need_bytes) __releases(&head->lock)
+					    __acquires(&head->lock)
 {
 	list_add_tail(&tic->t_queue, &head->waiters);