xfs: make xfs_btree_magic more generic

Right now the xfs_btree_magic() define takes only a cursor;
change this to take crc and btnum args to make it more generically
useful, and move to a function.

This will allow xfs_btree_init_block_int callers which don't
have a cursor to make use of the xfs_magics array, which will
happen in the next patch.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
diff --git a/fs/xfs/libxfs/xfs_btree.h b/fs/xfs/libxfs/xfs_btree.h
index b69b947..95ea6ed 100644
--- a/fs/xfs/libxfs/xfs_btree.h
+++ b/fs/xfs/libxfs/xfs_btree.h
@@ -76,6 +76,8 @@ union xfs_btree_rec {
 #define	XFS_BTNUM_RMAP	((xfs_btnum_t)XFS_BTNUM_RMAPi)
 #define	XFS_BTNUM_REFC	((xfs_btnum_t)XFS_BTNUM_REFCi)
 
+__uint32_t xfs_btree_magic(int crc, xfs_btnum_t btnum);
+
 /*
  * For logging record fields.
  */