lift compat definitions of mcast [sg]etsockopt requests into net/compat.h

We want to get rid of compat_mc_[sg]etsockopt() and to have that stuff
handled without compat_alloc_user_space(), extra copying through
userland, etc.  To do that we'll need ipv4 and ipv6 instances of
->compat_[sg]etsockopt() to manipulate the 32bit variants of mcast
requests, so we need to move the definitions of those out of net/compat.c
and into a public header.

This patch just does a mechanical move to include/net/compat.h

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>