inet: frags: refactor lowpan_net_frag_init()

We want to call lowpan_net_frag_init() earlier.
Similar to commit "inet: frags: refactor ipv6_frag_init()"

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 807f1844df4ac23594268fa9f41902d0549e92aa)
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>