NFSv4: Decode mounted_on_fileid attribute in getattr.

It is ignored if fileid is also requested. This will be used on referrals
(fs_locations).

Signed-off-by: Manoj Naik <manoj@almaden.ibm.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>