ceph: ceph_kick_flushing_caps needs the s_mutex

The mdsc->cap_dirty_lock is not held while walking the list in
ceph_kick_flushing_caps, which is not safe.

ceph_early_kick_flushing_caps does something similar, but the
s_mutex is held while it's called and I think that guards against
changes to the list.

Ensure we hold the s_mutex when calling ceph_kick_flushing_caps,
and add some clarifying comments.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: "Yan, Zheng" <zyan@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

fs/ceph/caps.c

@@ -2518,6 +2518,8 @@ void ceph_kick_flushing_caps(struct ceph_mds_client *mdsc,
 	struct ceph_cap *cap;
 	u64 oldest_flush_tid;
 
+	lockdep_assert_held(&session->s_mutex);
+
 	dout("kick_flushing_caps mds%d\n", session->s_mds);
 
 	spin_lock(&mdsc->cap_dirty_lock);