ksmbd: count all requests in req_running counter

This changes the semantics of req_running to count all in-flight
requests on a given connection, rather than the number of elements
in the conn->request list. The latter is used only in smb2_cancel,
and the counter is not used

Signed-off-by: Marios Makassikis <mmakassikis@freebox.fr>
Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>