mptcp: fix address removal logic in mptcp_pm_nl_rm_addr

mptcp: fix address removal logic in mptcp_pm_nl_rm_addr

Fix inverted WARN_ON_ONCE condition that prevented normal address
removal counter updates. The current code only executes decrement
logic when the counter is already 0 (abnormal state), while
normal removals (counter > 0) are ignored.

Signed-off-by: Gang Yan <yangang@kylinos.cn>
Fixes: 636113918508 ("mptcp: pm: remove '_nl' from mptcp_pm_nl_rm_addr_received")
Cc: stable@vger.kernel.org
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20251118-net-mptcp-misc-fixes-6-18-rc6-v1-10-806d3781c95f@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>