It looks like metapage_releasepage was making in invalid assumption that
the releasepage method would not be called on a dirty page.  Instead of
issuing a warning and releasing the metapage, it should return 0, indicating
that the private data for the page cannot be released.

I also realized that metapage_releasepage had the return code all wrong.  If
it is successful in releasing the private data, it should return 1, otherwise
it needs to return 0.

Lastly, there is no need to call wait_on_page_writeback, since
try_to_release_page will not call us with a page in writback state.

Signed-off-by: Dave Kleikamp <shaggy@austin.ibm.com>