mm: page_alloc: simplify drain_zone_pages by using min()

Instead of open-coding getting minimal value of two, just use min macro.
That is why it is there for.  While changing the function also change
type of batch local variable to match type of per_cpu_pages::batch
(which is int).

Signed-off-by: Michal Nazarewicz <mina86@mina86.com>
Acked-by: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>