mm: move debug code out of page_alloc.c

dump_page() and dump_vma() are not specific to page_alloc.c, move them out
so page_alloc.c won't turn into the unofficial debug repository.

Signed-off-by: Sasha Levin <sasha.levin@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>