vsock/test: Add test for a linear and non-linear skb getting coalesced

vsock/test: Add test for a linear and non-linear skb getting coalesced

Loopback transport can mangle data in rx queue when a linear skb is
followed by a small MSG_ZEROCOPY packet.

To exercise the logic, send out two packets: a weirdly sized one (to ensure
some spare tail room in the skb) and a zerocopy one that's small enough to
fit in the spare room of its predecessor. Then, wait for both to land in
the rx queue, and check the data received. Faulty packets merger manifests
itself by corrupting payload of the later packet.

Signed-off-by: Michal Luczaj <mhal@rbox.co>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Link: https://patch.msgid.link/20260113-vsock-recv-coalescence-v2-2-552b17837cf4@rbox.co
Signed-off-by: Jakub Kicinski <kuba@kernel.org>