mptcp: pm: in-kernel: rename 'addrs' to 'endpoints'

mptcp: pm: in-kernel: rename 'addrs' to 'endpoints'

A few variables linked to the in-kernel Path-Manager are confusing, and
it would help current and future developers, to clarify them.

One of them is 'addrs', which in fact represents the number of declared
endpoints, and not only the 'signal' endpoints.

No functional changes intended.

Reviewed-by: Geliang Tang <geliang@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20250925-net-next-mptcp-c-flag-laminar-v1-11-ad126cc47c6b@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>