mm/zswap.c: add __init to zswap_entry_cache_destroy()

zswap_entry_cache_destroy() is only called by __init init_zswap().

This patch also fixes function name zswap_entry_cache_ s/destory/destroy

Signed-off-by: Fabian Frederick <fabf@skynet.be>
Acked-by: Seth Jennings <sjennings@variantweb.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>