zsmalloc: correct fragile [kmap|kunmap]_atomic use

The kunmap_atomic should use virtual address getting by kmap_atomic.
However, some pieces of code in zsmalloc uses modified address, not the
one got by kmap_atomic for kunmap_atomic.

It's okay for working because zsmalloc modifies the address inner
PAGE_SIZE bounday so it works with current kmap_atomic's implementation.
But it's still fragile with potential changing of kmap_atomic so let's
correct it.

I got a subtle bug when I implemented a new feature of zsmalloc
(compaction) due to a link's mishandling (the link was over page
boundary).  Although it was totally my mistake, it took a while to find
the cause because an unpredictable kmapped address was unmapped causing an
almost random crash.

Signed-off-by: Minchan Kim <minchan@kernel.org>
Cc: Nitin Gupta <ngupta@vflare.org>
Cc: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Cc: Dan Streetman <ddstreet@ieee.org>
Cc: Seth Jennings <sjennings@variantweb.net>
Cc: Jerome Marchand <jmarchan@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>