inet: frags: refactor ipv6_frag_init()

We want to call inet_frags_init() earlier.

This is a prereq to "inet: frags: use rhashtables for reassembly units"

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
(cherry picked from commit 5b975bab23615cd0fdf67af6c9298eb01c4b9f61)
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>