mm/zswap: bugfix: memory leak when re-swapon

mm/zswap: bugfix: memory leak when re-swapon

zswap_tree is not freed when swapoff, and it got re-kmalloced in swapon,
so a memory leak occurs.

Free the memory of zswap_tree in zswap_frontswap_invalidate_area().

Signed-off-by: Weijie Yang <[email protected]>
Reviewed-by: Bob Liu <[email protected]>
Cc: Minchan Kim <[email protected]>
Reviewed-by: Minchan Kim <[email protected]>
Cc: <[email protected]>
From: Weijie Yang <[email protected]>
Subject: mm/zswap: bugfix: memory leak when invalidate and reclaim occur concurrently

Consider the following scenario:
thread 0: reclaim entry x (get refcount, but not call zswap_get_swap_cache_page)
thread 1: call zswap_frontswap_invalidate_page to invalidate entry x.
	finished, entry x and its zbud is not freed as its refcount != 0
	now, the swap_map[x] = 0
thread 0: now call zswap_get_swap_cache_page
	swapcache_prepare return -ENOENT because entry x is not used any more
	zswap_get_swap_cache_page return ZSWAP_SWAPCACHE_NOMEM
	zswap_writeback_entry do nothing except put refcount
Now, the memory of zswap_entry x and its zpage leak.

Modify:
 - check the refcount in fail path, free memory if it is not referenced.

 - use ZSWAP_SWAPCACHE_FAIL instead of ZSWAP_SWAPCACHE_NOMEM as the fail path
   can be not only caused by nomem but also by invalidate.

[[email protected]: coding-style fixes]
Signed-off-by: Weijie Yang <[email protected]>
Reviewed-by: Bob Liu <[email protected]>
Cc: Minchan Kim <[email protected]>
Cc: <[email protected]>
Acked-by: Seth Jennings <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>