xfs: set a default CoW extent size of 32 blocks

If the admin doesn't set a CoW extent size or a regular extent size
hint, default to creating CoW reservations 32 blocks long to reduce
fragmentation.

Signed-off-by: DarricK J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>

fs/xfs/xfs_inode.h

@@ -490,4 +490,7 @@ do { \
 
 extern struct kmem_zone	*xfs_inode_zone;
 
+/* The default CoW extent size hint. */
+#define XFS_DEFAULT_COWEXTSZ_HINT 32
+
 #endif	/* __XFS_INODE_H__ */