selftests/bpf: Add test for bpf_obj_drop with bad reg->off

Add a selftest for the fix provided in the previous commit. Without the
fix, the selftest passes the verifier while it should fail. The special
logic for detecting graph root or node for reg->off and bypassing
reg->off == 0 guarantee for release helpers/kfuncs has been dropped.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20230822175140.1317749-3-memxor@gmail.com


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