mm/page_alloc.c: simplify the code by using macro K()

mm/page_alloc.c: simplify the code by using macro K()

Use helper macro K() to convert the pages to the corresponding size.
Minor readability improvement.

Link: https://lkml.kernel.org/r/20210902121242.41607-3-linmiaohe@huawei.com
Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Acked-by: Mel Gorman <mgorman@techsingularity.net>
Reviewed-by: David Hildenbrand <david@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>