cifs: Fix in error types returned for out-of-credit situations.

For failure by timeout waiting for credits, changed the error
returned to the app with EBUSY, instead of ENOTSUPP. This is done
because this situation is possible even in non-buggy cases. i.e.
overloaded server can return 0 credits until done with outstanding
requests. And this feels like a better error to return to the app.

For cases of zero credits found even when there are no requests
in flight, replaced ENOTSUPP with EDEADLK, since we're avoiding
deadlock here by returning error.

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