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_extent_busy.c b/fs/xfs/xfs_extent_busy.c
index 86f559f..e43708e 100644
--- a/fs/xfs/xfs_extent_busy.c
+++ b/fs/xfs/xfs_extent_busy.c
@@ -160,7 +160,8 @@
 	struct xfs_extent_busy	*busyp,
 	xfs_agblock_t		fbno,
 	xfs_extlen_t		flen,
-	bool			userdata)
+	bool			userdata) __releases(&pag->pagb_lock)
+					  __acquires(&pag->pagb_lock)
 {
 	xfs_agblock_t		fend = fbno + flen;
 	xfs_agblock_t		bbno = busyp->bno;