NFSD: Fix offset type in I/O trace points

commit 6a4d333d

 upstream.

NFSv3 and NFSv4 use u64 offset values on the wire. Record these values
verbatim without the implicit type case to loff_t.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>