netfilter: ipset: exclude gc when resize is in progress

netfilter: ipset: exclude gc when resize is in progress

Zhengchuan Liang and Eulgyu Kim reported that because resize
does not copy the comment extension into the resized set but
uses it's pointer, ongoing gc can free the extension in the
original set which then results stale pointer in the resized
one. The proposed patch was to recreate the extensions for
every element in the resized set. It is both expensive and
wastes memory, so better exclude gc when resizing in progress
detected: resizing will destroy the original set anyway,
so doing gc on it is unnecessary.

Introduce a new spinlock to exclude parallel gc and resize.
Because we just set and check a bool value, there's no need
for the parameter to be atomic_t and rename it for better
readability.

Reported-by: Yuan Tan <yuantan098@gmail.com>
Reported-by: Yifan Wu <yifanwucs@gmail.com>
Reported-by: Juefei Pu <tomapufckgml@gmail.com>
Reported-by: Xin Liu <bird@lzu.edu.cn>
Reported by: Zhengchuan Liang <zcliangcn@gmail.com>
Reported by: Eulgyu Kim <eulgyukim@snu.ac.kr>
Signed-off-by: Jozsef Kadlecsik <kadlec@netfilter.org>
Signed-off-by: Florian Westphal <fw@strlen.de>