mptcp: sockopt: fix getting freebind & transparent

mptcp: sockopt: fix getting freebind & transparent

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

IP(V6)_FREEBIND and IP(V6)_TRANSPARENT 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.

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

Fixes: c9406a23c116 ("mptcp: sockopt: add SOL_IP freebind & transparent options")
Cc: stable@vger.kernel.org
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-v1-3-122dbb249db3@kernel.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>