UPSTREAM: zsmalloc: remove synchronize_rcu from zs_compact()

(cherry-pick from commit 1ec7cfb13acb8047ae5baafb43d2cd6b64ac85b9)

Do not synchronize rcu in zs_compact(). Neither zsmalloc not
zram use rcu.

Bug: 25951511

Change-Id: I92cc840e7eab0368832d7c6355933ae38c04c100
Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Acked-by: Minchan Kim <minchan@kernel.org>
Cc: Nitin Gupta <ngupta@vflare.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>