mptcp: Build-Fix

When fullmesh is enabled and IPv6 is disabled we had:

net/mptcp/mptcp_fullmesh.c: In function ‘full_mesh_new_session’:
net/mptcp/mptcp_fullmesh.c:1114:24: error: ‘struct sock_common’ has no member named ‘skc_v6_daddr’
make[3]: *** [net/mptcp/mptcp_fullmesh.o] Error 1
make[3]: *** Waiting for unfinished jobs....
make[2]: *** [net/mptcp] Error 2
make[1]: *** [net] Error 2
make: *** [sub-make] Error 2

Fixes: 2ac4542f96e65 (mptcp: Reduce mptcp_update_metasocket to its minimum)
Signed-off-by: Christoph Paasch <christoph.paasch@uclouvain.be>