mptcp: pm: only fill id_avail_bitmap for in-kernel pm

mptcp: pm: only fill id_avail_bitmap for in-kernel pm

id_avail_bitmap of struct mptcp_pm_data is currently only used by the
in-kernel PM, so this patch moves its initialization operation under
the "if (pm_type == MPTCP_PM_TYPE_KERNEL)" condition.

Suggested-by: Matthieu Baerts <matttbe@kernel.org>
Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250313-net-next-mptcp-pm-ops-intro-v1-4-f4e4a88efc50@kernel.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>