mm: pagewalk: fix the comment for test_walk

Modify the comment describing struct mm_walk->test_walk()s behaviour to
match the comment on walk_page_test() and the behaviour of
walk_page_vma().

Fixes: fafaa426

 ("pagewalk: improve vma handling")
Link: http://lkml.kernel.org/r/1471622518-21980-1-git-send-email-james.morse@arm.com
Signed-off-by: James Morse <james.morse@arm.com>
Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>