mptcp: constify a bunch of of helpers

commit 90d93088 upstream.

A few pm-related helpers don't touch arguments which lacking
the const modifier, let's constify them.

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>
Stable-dep-of: 48e50dcb ("mptcp: pm: avoid possible UaF when selecting endp")
[ Conflicts because a few modified helpers from the original patch are
  not present in this version. We don't need to do anything with them. ]
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>