NFSv4: GETATTR attributes on referral

Per referral draft, only fs_locations, fsid, and mounted_on_fileid can be
requested in a GETATTR on referrals.

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