ipv6: do not check fib6_has_expires() in fib6_info_release()

My prior patch went a bit too far, because apparently fib6_has_expires()
could be true while f6i->gc_link is not hashed yet.

fib6_set_expires_locked() can indeed set RTF_EXPIRES
while f6i->fib6_table is NULL.

Original syzbot reports were about corruptions caused
by dangling f6i->gc_link.

Fixes: 5a08d006

 ("ipv6: add debug checks in fib6_info_release()")
Reported-by:  <syzbot+c15aa445274af8674f41@syzkaller.appspotmail.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Kui-Feng Lee <thinker.li@gmail.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/20231207201322.549000-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>