This worker function is needed to send SMB2 fsctl
(and ioctl) requests including:

validating negotiation info (secure negotiate)
querying the servers network interfaces
copy offload (refcopy)

Followon patches for the above three will use this.
This patch also does general validation of the response.

In the future, as David Disseldorp notes, for the copychunk ioctl
case, we will want to enhance the response processing to allow
returning the chunk request limits to the caller (even
though the server returns an error, in that case we would
return data that the caller could use - see 2.2.32.1).

See MS-SMB2 Section 2.2.31 for more details on format of fsctl.
Acked-by: Pavel Shilovsky <piastry@etersoft.ru>
Signed-off-by: Steve French <smfrench@gmail.com>