Unlock before returning an error code if this allocation fails.

Fixes: 0626e664 ("cifsd: add server handler for central processing and tranport layers")
Cc: stable@vger.kernel.org # v5.15+
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>