cifs: add a warning when the in-flight count goes negative

We've seen the in-flight count go into negative with some
internal stress testing in Microsoft.

Adding a WARN when this happens, in hope of understanding
why this happens when it happens.
Signed-off-by: Shyam Prasad N <sprasad@microsoft.com>
Reviewed-by: Bharath SM <bharathsm@microsoft.com>
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/smb/client/smb2ops.c

@@ -93,6 +93,7 @@ smb2_add_credits(struct TCP_Server_Info *server,
 					    server->conn_id, server->hostname, *val,
 					    add, server->in_flight);
 	}
+	WARN_ON_ONCE(server->in_flight == 0);
 	server->in_flight--;
 	if (server->in_flight == 0 &&
 	   ((optype & CIFS_OP_MASK) != CIFS_NEG_OP) &&