CIFS: Do not assume one credit for async responses

[ Upstream commit 0fd1d37b0501efc6e295f56ab55cdaff784aa50c ]

If we don't receive a response we can't assume that the server
granted one credit. Assume zero credits in such cases.

Signed-off-by: Pavel Shilovsky <pshilov@microsoft.com>
Reviewed-by: Ronnie Sahlberg <lsahlber@redhat.com>
Signed-off-by: Steve French <stfrench@microsoft.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>