nfs: add LAYOUT_TYPE_MAX enum value

This gives us a nice upper bound for later use in nfѕd.
Signed-off-by: NChristoph Hellwig <hch@lst.de>

include/linux/nfs4.h

@@ -516,6 +516,7 @@ enum pnfs_layouttype {
 	LAYOUT_NFSV4_1_FILES  = 1,
 	LAYOUT_OSD2_OBJECTS = 2,
 	LAYOUT_BLOCK_VOLUME = 3,
+	LAYOUT_TYPE_MAX
 };
 
 /* used for both layout return and recall */