nfsd: clean up an xdr reserved space calculation

We should use XDR_LEN to calculate reserved space in case the oid is not
a multiple of 4.

RESERVE_SPACE actually rounds up for us, but it's probably better to be
careful here.

Signed-off-by: Kinglong Mee <kinglongmee@gmail.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>