memcg: remove experimental from swap account config

memcg: remove experimental from swap account config

It's 11 months since we changed swap_map[] to indicates SWAP_HAS_CACHE.
Since that, memcg's swap accounting has been very stable and it seems
it can be maintained.

So, I'd like to remove EXPERIMENTAL from the config.

Acked-by: Balbir Singh <[email protected]>
Acked-by: Daisuke Nishimura <[email protected]>
Signed-off-by: KAMEZAWA Hiroyuki <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>