mptcp: export local_address

commit dc886bce upstream.

Rename local_address() with "mptcp_" prefix and export it in protocol.h.

This function will be re-used in the common PM code (pm.c) in the
following commit.

Signed-off-by: Geliang Tang <geliang.tang@suse.com>
Reviewed-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Reviewed-by: Larysa Zaremba <larysa.zaremba@intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Stable-dep-of: 6834097f ("mptcp: pm: fix backup support in signal endpoints")
[ Conflicts in pm_netlink.c and protocol.h, because the context has
  changed in commit 4638de5a ("mptcp: handle local addrs announced
  by userspace PMs") which is not in this version. This commit is
  unrelated to this modification. ]
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>