ceph: refactor ceph_write_begin, fix ceph_page_mkwrite

Originally ceph_page_mkwrite called ceph_write_begin, hoping that
the returned locked page would be the page that it was requested
to mkwrite. Factored out relevant part of ceph_page_mkwrite and
we lock the right page anyway.

Signed-off-by: Yehuda Sadeh <yehuda@hq.newdream.net>
Signed-off-by: Sage Weil <sage@newdream.net>