ofproto-dpif-xlate: Make code more readable via 'flow' and 'wc' locals.

ofproto-dpif-xlate: Make code more readable via 'flow' and 'wc' locals.

'ctx->xin->flow' and 'ctx->xout->wc' are both pretty long.  Where it gets
in the way of code readability, this patch replaces them by 'xin' and
'xout' using local variables.

Also, replace an explicit comparison against IP and IPv6 Ethertypes by
a call to is_ip_any().

Co-authored-by: Jarno Rajahalme <jarno.rajahalme@nsn.com>.
Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Ethan Jackson <ethan@nicira.com>