mm: MADV_FREE trivial clean up

mm: MADV_FREE trivial clean up

1. Page table waker already pass the vma it is processing
so we don't need to pass vma.

2. If page table entry is dirty in try_to_unmap_one, the dirtiness
should propagate to PG_dirty of the page. So, it's enough to check
only PageDirty without other pte dirty bit checking.

Signed-off-by: Minchan Kim <minchan@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>