ceph: fix capsnap dirty pages accounting

writepages_finish() calls ceph_put_wrbuffer_cap_refs() once for
all pages, parameter snapc is set to req->r_snapc. So writepages()
shouldn't write dirty pages associated with different snapc in
one OSD request.

Signed-off-by: "Yan, Zheng" <zyan@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>