mm/zbud.c: use list_last_entry() instead of list_tail_entry()

list_last_entry*( has been defined in list.h, so replace
list_tail_entry() with it.

Signed-off-by: Geliang Tang <geliangtang@163.com>
Cc: Seth Jennings <sjennings@variantweb.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>