[IPV6]: Cleanup of net/ipv6/reassambly.c

Two minor cleanups:

1. Using kzalloc() in fraq_alloc_queue()
   saves the memset() in ipv6_frag_create().

2. Invert sense of if-statements to streamline code.
   Inverts the comment, too.

Signed-off-by: Ingo Oeser <ioe-lkml@rameria.de>
Signed-off-by: David S. Miller <davem@davemloft.net>