xfs: Use defines for CRC offsets in all cases

Some calls to crc functions used useful #defines,
others used awkward offsetof() constructs.

Switch them all to #define to make things a bit cleaner.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>

diff --git a/fs/xfs/xfs_sb.h b/fs/xfs/xfs_sb.h
index 35061d4..f7b2fe7 100644
--- a/fs/xfs/xfs_sb.h
+++ b/fs/xfs/xfs_sb.h
@@ -182,6 +182,8 @@
 	/* must be padded to 64 bit alignment */
 } xfs_sb_t;
 
+#define XFS_SB_CRC_OFF		offsetof(struct xfs_sb, sb_crc)
+
 /*
  * Superblock - on disk version.  Must match the in core version above.
  * Must be padded to 64 bit alignment.