mm/page_alloc: make pcpu_drain_mutex and pcpu_drain static

mm/page_alloc: make pcpu_drain_mutex and pcpu_drain static

Fix the following sparse warning:

  mm/page_alloc.c:106:1: warning: symbol 'pcpu_drain_mutex' was not declared. Should it be static?
  mm/page_alloc.c:107:1: warning: symbol '__pcpu_scope_pcpu_drain' was not declared. Should it be static?

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Link: http://lkml.kernel.org/r/20200407023925.46438-1-yanaijie@huawei.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>