expr: limit: operational limit match

The nft limit match currently does not work at all.  Below patches to
nftables, libnftables, and kernel address the issue.  A few notes on
the implementation:

- Removed support for nano/micro/milli second limits.  These seem pointless,
  given we are using jiffies in the limit match, not a hpet.  And who really
  needs to limit items down to sub-second level??

- 'depth' member is removed as unnecessary.  All we need in the kernel is the
  rate and the unit.

- 'stamp' member becomes the time we need to next refresh the token bucket,
  instead of being updated on every packet which goes through the match.

This closes netfilter bugzilla #827, reported by Eric Leblond.

Signed-off-by: Phil Oester <kernel@linuxace.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>