ipv6: frags: rewrite ip6_expire_frag_queue()

Make it similar to IPv4 ip_expire(), and release the lock
before calling icmp functions.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
(cherry picked from commit 05c0b86b)
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>