ceph: fix ci->i_head_snapc leak

We missed two places that i_wrbuffer_ref_head, i_wr_ref, i_dirty_caps
and i_flushing_caps may change. When they are all zeros, we should free
i_head_snapc.

Cc: [e-mail]
Link: https://tracker.ceph.com/issues/38224
Reported-and-tested-by: Luis Henriques <[e-mail]>
Signed-off-by: "Yan, Zheng" <[e-mail]>
Signed-off-by: Ilya Dryomov <[e-mail]>

fs/ceph/mds_client.c

···
 			list_add(&ci->i_prealloc_cap_flush->i_list, &to_remove);
 			ci->i_prealloc_cap_flush = NULL;
 		}
+
+               if (drop &&
+                  ci->i_wrbuffer_ref_head == 0 &&
+                  ci->i_wr_ref == 0 &&
+                  ci->i_dirty_caps == 0 &&
+                  ci->i_flushing_caps == 0) {
+                      ceph_put_snap_context(ci->i_head_snapc);
+                      ci->i_head_snapc = NULL;
+               }
 	}
 	spin_unlock(&ci->i_ceph_lock);
 	while (!list_empty(&to_remove)) {

fs/ceph/snap.c

···
 	old_snapc = NULL;
 
 update_snapc:
-	if (ci->i_head_snapc) {
+       if (ci->i_wrbuffer_ref_head == 0 &&
+           ci->i_wr_ref == 0 &&
+           ci->i_dirty_caps == 0 &&
+           ci->i_flushing_caps == 0) {
+               ci->i_head_snapc = NULL;
+       } else {
 		ci->i_head_snapc = ceph_get_snap_context(new_snapc);
 		dout(" new snapc is %p\n", new_snapc);
 	}