mptcp: Align mptcp_inet6_sk with other protocols

mptcp: Align mptcp_inet6_sk with other protocols

Ever since commit f5f80e32de12 ("ipv6: remove hard coded limitation on
ipv6_pinfo") that protocols stopped using the old "obj_size -
sizeof(struct ipv6_pinfo)" way of grabbing ipv6_pinfo, that severely
restricted struct layout and caused fun, hard to see issues.

However, mptcp_inet6_sk wasn't fixed (unlike tcp_inet6_sk). Do so.
The non-cloned sockets already do the right thing using
ipv6_pinfo_offset + the generic IPv6 code.

Signed-off-by: Pedro Falcato <pfalcato@suse.de>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20250430154541.1038561-1-pfalcato@suse.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>