mm: memory: merge shared-writable dirtying branches in do_wp_page()

Whether there is a vm_ops->page_mkwrite or not, the page dirtying is
pretty much the same.  Make sure the page references are the same in both
cases, then merge the two branches.

It's tempting to go even further and page-lock the !page_mkwrite case, to
get it in line with everybody else setting the page table and thus further
simplify the model.  But that's not quite compelling enough to justify
dropping the pte lock, then relocking and verifying the entry for
filesystems without ->page_mkwrite, which notably includes tmpfs.  Leave
it for now and lock the page late in the !page_mkwrite case.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>