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

fs/ceph/addr.c

@@ -930,8 +930,8 @@ static int ceph_writepages_start(struct address_space *mapping,
 
 			/* only if matching snap context */
 			pgsnapc = page_snap_context(page);
-			if (pgsnapc->seq > snapc->seq) {
-				dout("page snapc %p %lld > oldest %p %lld\n",
+			if (pgsnapc != snapc) {
+				dout("page snapc %p %lld != oldest %p %lld\n",
 				     pgsnapc, pgsnapc->seq, snapc, snapc->seq);
 				unlock_page(page);
 				continue;