libceph: prevent the race of incoming work during teardown

Add an atomic variable 'stopping' as flag in struct ceph_messenger,
set this flag to 1 in function ceph_destroy_client(), and add the condition code
in function ceph_data_ready() to test the flag value, if true(1), just return.

Signed-off-by: Guanjun He <gjhe@suse.com>
Reviewed-by: Sage Weil <sage@inktank.com>

net/ceph/ceph_common.c

@@ -495,6 +495,8 @@ void ceph_destroy_client(struct ceph_client *client)
 {
 	dout("destroy_client %p\n", client);
 
+	atomic_set(&client->msgr.stopping, 1);
+
 	/* unmount */
 	ceph_osdc_stop(&client->osdc);