nfs: fix v4.2 SEEK on files over 2 gigs

We're incorrectly assigning a loff_t return to an int.  If SEEK_HOLE or
SEEK_DATA returns an offset over 2^31 then the application will see a
weird lseek() result (usually -EIO).

Cc: stable@vger.kernel.org
Fixes: bdcc2cd1

 "NFSv4.2: handle NFS-specific llseek errors"
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Reviewed-by: Anna Schumaker <Anna.Schumaker@Netapp.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>