fs: add new FMODE flags: FMODE_32bithash and FMODE_64bithash

Those flags are supposed to be set by NFS readdir() to tell ext3/ext4
to 32bit (NFSv2) or 64bit hash values (offsets) in seekdir().

Signed-off-by: Bernd Schubert <bernd.schubert@itwm.fraunhofer.de>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>