CIFS: Move credit processing to mid callbacks for SMB3

Currently we account for credits in the thread initiating a request
and waiting for a response. The demultiplex thread receives the response,
wakes up the thread and the latter collects credits from the response
buffer and add them to the server structure on the client. This approach
is not accurate, because it may race with reconnect events in the
demultiplex thread which resets the number of credits.

Fix this by moving credit processing to new mid callbacks that collect
credits granted by the server from the response in the demultiplex thread.

Signed-off-by: Pavel Shilovsky <pshilov@microsoft.com>
Signed-off-by: Steve French <stfrench@microsoft.com>