mm: reduce dependencies on <linux/kernel.h>

mm: reduce dependencies on <linux/kernel.h>

"page_counter.h" does not need <linux/kernel.h>. <linux/limits.h> is enough
to get LONG_MAX.

Files that include page_counter.h are limited. They have been compile
tested or checked.

$ git grep page_counter\.h
include/linux/hugetlb_cgroup.h: struct page_counter hugepage[HUGE_MAX_HSTATE];
	--> all files that include it have been compile tested

include/linux/memcontrol.h:#include <linux/page_counter.h>
	--> <linux/kernel.h> has been added, to be safe

include/net/sock.h:#include <linux/page_counter.h>
	--> already include <linux/kernel.h>

mm/hugetlb_cgroup.c:#include <linux/page_counter.h>
mm/memcontrol.c:#include <linux/page_counter.h>
mm/page_counter.c:#include <linux/page_counter.h>
	--> compile tested

Link: https://lkml.kernel.org/r/adfdbe21c4d06400d7bd802868762deb85cae8b6.1706908921.git.christophe.jaillet@wanadoo.fr
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>