usbip: add USBIP_URB_* URB transfer flags

USBIP driver packs URB transfer flags in network packets that are
exchanged between Server (usbip_host) and Client (vhci_hcd).

URB_* flags are internal to kernel and could change. Where as USBIP
URB flags exchanged in network packets are USBIP user API must not
change.

Add USBIP_URB* flags to make this an explicit API and change the
client and server to map them. Details as follows:

Client tx path (USBIP_CMD_SUBMIT):
- Maps URB_* to USBIP_URB_* when it sends USBIP_CMD_SUBMIT packet.

Server rx path (USBIP_CMD_SUBMIT):
- Maps USBIP_URB_* to URB_* when it receives USBIP_CMD_SUBMIT packet.

Flags aren't included in USBIP_CMD_UNLINK and USBIP_RET_SUBMIT packets
and no special handling is needed for them in the following cases:

- Server rx path (USBIP_CMD_UNLINK)
- Client rx path & Server tx path (USBIP_RET_SUBMIT)

Update protocol documentation to reflect the change.

Suggested-by: Hongren Zenithal Zheng <i@zenithal.me>
Suggested-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>
Link: https://lore.kernel.org/r/20220824002456.94605-1-skhan@linuxfoundation.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

docs: usbip: Fix major fields and descriptions in protocol

The old document for usbip protocol is misleading and hard to read:
* Some fields in header are incorrect
* Explanation of some fields are unclear or even wrong
* Padding of header (namely all headers have the same length) is
not explicitly pointed out, which is crucial for stream protocol
like TCP

Major changes:
* Document the correct field as described in the codebase.
* Document the padding in usbip headers. This is crucial for TCP
stream hence these padding should be explicitly point out.
In code these padding are implemented by a union of all headers.
* Fix two FIXME related to usbip unlink and Document the behavior
of unlink in different situation.
* Clarify some field with more accurate explanation, like those
fields associated with URB. Some constraints are extracted from
code.
* Delete specific transfer_flag doc in usbip as it should be
documented by the URB doc in Documentation/driver-api/usb/URB.rst
* Add data captured from wire as example

Version change:
From "PRELIMINARY DRAFT, MAY CONTAIN MISTAKES, 28 Jun 2011"
To "Version 1, 31 Mar 2021"

Co-developed-by: Alexandre Demers <alexandre.f.demers@gmail.com>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Shuah Khan <skhan@linuxfoundation.org>
Signed-off-by: Hongren Zheng <i@zenithal.me>
Link: https://lore.kernel.org/r/YGNZHfmAbHO6fyAr@Sun
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

docs: usb: rename files to .rst and add them to drivers-api

While there are a mix of things here, most of the stuff
were written from Kernel developer's PoV. So, add them to
the driver-api book.

A follow up for this patch would be to move documents from
there that are specific to sysadmins, adding them to the
admin-guide.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Acked-by: Johan Hovold <johan@kernel.org>
Acked-by: Felipe Balbi <felipe.balbi@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>