netfilter: nft_quota: use atomic64_xchg for reset

netfilter: nft_quota: use atomic64_xchg for reset

Use atomic64_xchg() to atomically read and zero the consumed value
on reset, which is simpler than the previous read+sub pattern and
doesn't require lock serialization.

Fixes: bd662c4218f9 ("netfilter: nf_tables: Add locking for NFT_MSG_GETOBJ_RESET requests")
Fixes: 3d483faa6663 ("netfilter: nf_tables: Add locking for NFT_MSG_GETSETELEM_RESET requests")
Fixes: 3cb03edb4de3 ("netfilter: nf_tables: Add locking for NFT_MSG_GETRULE_RESET requests")
Suggested-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Brian Witte <brianwitte@mailfence.com>
Signed-off-by: Florian Westphal <fw@strlen.de>