mm/z3fold.c: use xx_zalloc instead xx_alloc and memset

alloc_slots() allocates memory for slots using kmem_cache_alloc(), then
memsets it.  We can just use kmem_cache_zalloc().

Signed-off-by: Hui Su <sh_def@163.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Link: https://lkml.kernel.org/r/20200926100834.GA184671@rlk


Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>