mm/swap.c: put activate_page_pvecs and other pagevecs together

Put the activate_page_pvecs definition next to those of the other
pagevecs, for clarity.

Signed-off-by: Ming Li <mingli199x@qq.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>