vsock/test: Add test for MSG_ZEROCOPY completion memory leak

Exercise the ENOMEM error path by attempting to hit net.core.optmem_max
limit on send().

Test aims to create a memory leak, kmemleak should be employed.

Fixed by commit 60cf6206 ("virtio/vsock: Improve MSG_ZEROCOPY error
handling").

Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Michal Luczaj <mhal@rbox.co>
Link: https://patch.msgid.link/20241219-test-vsock-leaks-v4-7-a416e554d9d7@rbox.co


Signed-off-by: Jakub Kicinski <kuba@kernel.org>