mptcp: Call dst_release() in mptcp_active_enable().

mptcp: Call dst_release() in mptcp_active_enable().

mptcp_active_enable() calls sk_dst_get(), which returns dst with its
refcount bumped, but forgot dst_release().

Let's add missing dst_release().

Cc: stable@vger.kernel.org
Fixes: 27069e7cb3d1 ("mptcp: disable active MPTCP in case of blackhole")
Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20250916214758.650211-7-kuniyu@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>