cifs: fix max_credits implementation

The current implementation of max_credits on the client does
not work because the CreditRequest logic for several commands
does not take max_credits into account.

Still, we can end up asking the server for more credits, depending
on the number of credits in flight. For this, we need to
limit the credits while parsing the responses too.

Signed-off-by: Shyam Prasad N <sprasad@microsoft.com>
Signed-off-by: Steve French <stfrench@microsoft.com>