selftests/bpf: Add test for stale bounds on LSM retval context load

selftests/bpf: Add test for stale bounds on LSM retval context load

Add a verifier test that catches the stale-bounds issue fixed in the
previous patch. The test sets r6 = 0 to create known bounds, then loads
the LSM hook return value into r6 from the context. Without the fix,
the verifier intersects the retval range with the stale bounds and
incorrectly narrows r6 to a single value, pruning the fall-through
branch as dead code and missing the div-by-zero.

Suggested-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Tristan Madani <tristan@talencesecurity.com>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20260622230123.3695446-3-tristmd@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>