smb: client: make use of smbdirect_socket.recv_io.credits.available

In the Linux kernel, the following vulnerability has been resolved:

smb: client: make use of smbdirect_socket.recv_io.credits.available

The logic off managing recv credits by counting posted recv_io and

granted credits is racy.

That's because the peer might already consumed a credit,

but between receiving the incoming recv at the hardware

and processing the completion in the 'recv_done' functions

we likely have a window where we grant credits, which

don't really exist.

So we better have a decicated counter for the

available credits, which will be incremented

when we posted new recv buffers and drained when

we grant the credits to the peer.