mm/zswap.c: update zsmalloc in comment to zbud

zswap used zsmalloc before and now using zbud.  But, some comments saying
it use zsmalloc yet.  Fix the trivial problems.

Signed-off-by: SeongJae Park <sj38.park@gmail.com>
Cc: Seth Jennings <sjenning@linux.vnet.ibm.com>
Cc: Minchan Kim <minchan@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>