Replaces SetPageDirty() with set_page_dirty() in several places related
to in-memory filesystems.

SetPageDirty() is basically always the wrong thing to do.  Pages should
be moved to the ->dirty_pages list when dirtied so that writeback can
see them.

Without this change, dirty pages against in-memory filesystems would
churn around on the inactive list all the time, rather than getting
pushed away onto the active list.  A minor efficiency thing.