cifs: add a warning if we try to to dequeue a deleted mid

cifs_delete_mid() is called once we are finished handling a mid and we
expect no more work done on this mid.

Needed to fix recent commit:
commit 730928c8f4
("cifs: update smb2_queryfs() to use compounding")

Add a warning if someone tries to dequeue a mid that has already been
flagged to be deleted.
Also change list_del() to list_del_init() so that if we have similar bugs
resurface in the future we will not oops.

Signed-off-by: Ronnie Sahlberg <lsahlber@redhat.com>
Signed-off-by: Steve French <stfrench@microsoft.com>
Reviewed-by: Pavel Shilovsky <pshilov@microsoft.com>

fs/cifs/cifsglob.h

@@ -1553,6 +1553,7 @@ static inline void free_dfs_info_array(struct dfs_info3_param *param,
 
 /* Flags */
 #define   MID_WAIT_CANCELLED	 1 /* Cancelled while waiting for response */
+#define   MID_DELETED            2 /* Mid has been dequeued/deleted */
 
 /* Types of response buffer returned from SendReceive2 */
 #define   CIFS_NO_BUFFER        0    /* Response buffer not returned */