nfsd: document new filehandle fsid types

Descriptions taken from mountd code (in nfs-utils/utils/mountd/cache.c).
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>

include/linux/nfsd/nfsfh.h

@@ -68,6 +68,10 @@ struct nfs_fhbase_old {
  *     1  - 4 byte user specified identifier
  *     2  - 4 byte major, 4 byte minor, 4 byte inode number - DEPRECATED
  *     3  - 4 byte device id, encoded for user-space, 4 byte inode number
+ *     4  - 4 byte inode number and 4 byte uuid
+ *     5  - 8 byte uuid
+ *     6  - 16 byte uuid
+ *     7  - 8 byte inode number and 16 byte uuid
  *
  * The fileid_type identified how the file within the filesystem is encoded.
  * This is (will be) passed to, and set by, the underlying filesystem if it supports