mm: softdirty: don't forget to save file map softdiry bit on unmap

mm: softdirty: don't forget to save file map softdiry bit on unmap

pte_file_mksoft_dirty operates with argument passed by a value and returns
modified result thus we need to assign @ptfile here, otherwise itis a no-op
which may lead to loss of the softdirty bit.

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Cc: Pavel Emelyanov <xemul@parallels.com>
Cc: Hugh Dickins <hughd@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>