ksmbd: send pending interim for last compound I/O

ksmbd: send pending interim for last compound I/O

smb2.compound_async.write_write and smb2.compound_async.read_read expect
the last I/O request in a compound request to become cancellable before
its final response is received. smb clients mark a request cancellable
after receiving an interim STATUS_PENDING response.

ksmbd handled the last READ/WRITE synchronously and returned the final
response directly, so the client never observed STATUS_PENDING and
req->cancel.can_cancel remained false.

For the last READ or WRITE in a compound request, register the work briefly
as async and send a STATUS_PENDING interim response before continuing with
the normal synchronous completion. The final READ/WRITE response remains
unchanged.

Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>