Comment 3 for bug 1841704

The bug was introduced by the following commit backported from the upstream stable 4.19.y:
https://git.launchpad.net/~ubuntu-kernel/ubuntu/+source/linux/+git/bionic/commit/?h=master-next&id=c779b6c6b215bc3b3b7fcb39ab62f816bd6dfb75

The mainline commit changes also test_verifier.c from bpf self_tests:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=4f7b3e82589e0de723780198ec7983e427144c0a

However, changing the expected error message for test #391 seems to be not enough, since 391/u failed with a different error message (R1 has unknown scalar with mixed signed bounds, pointer arithmetic with it prohibited for !root).