mm/zsmalloc.c: fix build when CONFIG_COMPACTION=n

Fixes: 701d6785

 ("mm/zsmalloc.c: fix race condition in zs_destroy_pool")
Link: http://lkml.kernel.org/r/201908251039.5oSbEEUT%25lkp@intel.com
Reported-by: kbuild test robot <lkp@intel.com>
Cc: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Cc: Henry Burns <henrywolfeburns@gmail.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Shakeel Butt <shakeelb@google.com>
Cc: Jonathan Adams <jwadams@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>