netfilter: nf_tables: Add locking for NFT_MSG_GETSETELEM_RESET requests

Set expressions' dump callbacks are not concurrency-safe per-se with
reset bit set. If two CPUs reset the same element at the same time,
values may underrun at least with element-attached counters and quotas.

Prevent this by introducing dedicated callbacks for nfnetlink and the
asynchronous dump handling to serialize access.

Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>