First step to move eviction handling into a work queue.

We lose two spots that accounted evicted fragments in MIB counters.

Accounting will be restored since the upcoming work-queue evictor
invokes the frag queue timer callbacks instead.
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>