payload: take endianess into account when updating the payload context

payload: take endianess into account when updating the payload context

payload_expr_pctx_update() uses the numeric protocol value in host byte
order to find the upper layer protocol. This obviously doesn't work for
protocol expressions in other byte orders, such as the ethernet protocol
on little endian.

Export the protocol value in the correct byte order and use that value
to look up the upper layer protocol.

Signed-off-by: Patrick McHardy <kaber@trash.net>