Merge branch 'seltests/xsk: prepare for AF_XDP multi-buffer testing'

Magnus Karlsson says:

====================
Prepare the AF_XDP selftests test framework code for the upcoming
multi-buffer support in AF_XDP. This so that the multi-buffer patch
set does not become way too large. In that upcoming patch set, we are
only including the multi-buffer tests together with any framework
code that depends on the new options bit introduced in the AF_XDP
multi-buffer implementation itself.

Currently, the test framework is based on the premise that a packet
consists of a single fragment and thus occupies a single buffer and a
single descriptor. Multi-buffer breaks this assumption, as that is the
whole purpose of it. Now, a packet can consist of multiple buffers and
therefore consume multiple descriptors.

The patch set starts with some clean-ups and simplifications followed
by patches that make sure that the current code works even when a
packet occupies multiple buffers. The actual code for sending and
receiving multi-buffer packets will be included in the AF_XDP
multi-buffer patch set as it depends on a new bit being used in the
options field of the descriptor.

Patch set anatomy:
1: The XDP program was unnecessarily changed many times. Fixes this.

2: There is no reason to generate a full UDP/IPv4 packet as it is
   never used. Simplify the code by just generating a valid Ethernet
   frame.

3: Introduce a more complicated payload pattern that can detect
   fragments out of bounds in a multi-buffer packet and other errors
   found in single-fragment packets.

4: As a convenience, dump the content of the faulty packet at error.

5: To simplify the code, make the usage of the packet stream for Tx
   and Rx more similar.

6: Store the offset of the packet in the buffer in the struct pkt
   definition instead of the address in the umem itself and introduce
   a simple buffer allocator. The address only made sense when all
   packets consumed a single buffer. Now, we do not know beforehand
   how many buffers a packet will consume, so we instead just allocate
   a buffer from the allocator and specify the offset within that
   buffer.

7: Test for huge pages only once instead of before each test that needs it.

8: Populate the fill ring based on how many frags are needed for each
   packet.

9: Change the data generation code so it can generate data for
   multi-buffer packets too.

10: Adjust the packet pacing algorithm so that it can cope with
    multi-buffer packets. The pacing algorithm is present so that Tx
    does not send too many packets/frames to Rx that it starts to drop
    packets. That would ruin the tests.

v1 -> v2:
* Fixed spelling error in patch #6 [Simon]
* Fixed compilation error with llvm in patch #7 [Daniel]

Thanks: Magnus
====================

Signed-off-by: Alexei Starovoitov <ast@kernel.org>