NFSD: Optimize nfsd4_encode_operation()

write_bytes_to_xdr_buf() is a generic way to place a variable-length
data item in an already-reserved spot in the encoding buffer.
However, it is costly, and here, it is unnecessary because the
data item is fixed in size, the buffer destination address is
always word-aligned, and the destination location is already in
@p.

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