NFSD: Use xdr_pad_size()

[ Upstream commit 5e64d85c ]

Clean up: Use a helper instead of open-coding the calculation of
the XDR pad size.

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>