xfs: refcount btree add more reserved blocks

Since XFS reserves a small amount of space in each AG as the minimum
free space needed for an operation, save some more space in case we
touch the refcount btree.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
diff --git a/fs/xfs/libxfs/xfs_format.h b/fs/xfs/libxfs/xfs_format.h
index 57c52a6..622055b 100644
--- a/fs/xfs/libxfs/xfs_format.h
+++ b/fs/xfs/libxfs/xfs_format.h
@@ -1455,6 +1455,8 @@
  */
 #define	XFS_REFC_CRC_MAGIC	0x52334643	/* 'R3FC' */
 
+unsigned int xfs_refc_block(struct xfs_mount *mp);
+
 
 /*
  * BMAP Btree format definitions