mptcp: sockopt: fix getting IPV6_V6ONLY

commit 8c396337 upstream.

When adding a socket option support in MPTCP, both the get and set parts
are supposed to be implemented.

IPV6_V6ONLY support for the setsockopt part has been added a while ago,
but it looks like the get part got forgotten. It should have been
present as a way to verify a setting has been set as expected, and not
to act differently from TCP or any other socket types.

Not supporting this getsockopt(IPV6_V6ONLY) blocks some apps which want
to check the default value, before doing extra actions. On Linux, the
default value is 0, but this can be changed with the net.ipv6.bindv6only
sysctl knob. On Windows, it is set to 1 by default. So supporting the
get part, like for all other socket options, is important.

Everything was in place to expose it, just the last step was missing.
Only new code is added to cover this specific getsockopt(), that seems
safe.

Fixes: c9b95a13 ("mptcp: support IPV6_V6ONLY setsockopt")
Cc: stable@vger.kernel.org
Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/550


Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250314-net-mptcp-fix-data-stream-corr-sockopt-v1-2-122dbb249db3@kernel.org


Signed-off-by: Paolo Abeni <pabeni@redhat.com>
[ Conflicts in sockopt.c in the context, because commit 0abdde82
  ("mptcp: move sockopt function into a new file") is not in this
  release. The modifications can still be done in protocol.c without
  difficulties. A particularity is that the mptcp_put_int_option()
  helper is required, and imported from newer versions without taking
  the extra features introduced with them in commit 2c9e7765
  ("mptcp: add TCP_INQ cmsg support") and commit 3b1e21eb ("mptcp:
  getsockopt: add support for IP_TOS"). ]
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>