mptcp: pm: simplify select_local_address()

There is no need to unconditionally acquire the join list
lock, we can simply splice the join list into the subflow
list and traverse only the latter.

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>