mptcp: use mptcp_set_state

This patch replaces all the 'inet_sk_state_store()' calls under net/mptcp
with the new helper mptcp_set_state().

Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/460


Signed-off-by: Geliang Tang <geliang.tang@linux.dev>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Matthieu Baerts <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts <matttbe@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>