bpf: Improve ctx access verifier error message

We've already had two "error during ctx access conversion" warnings
triggered by syzkaller. Let's improve the error message by dumping the
cnt variable so that we can more easily differentiate between the
different error cases.

Signed-off-by: Paul Chaignon <paul.chaignon@gmail.com>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/cc94316c30dd76fae4a75a664b61a2dbfe68e205.1754039605.git.paul.chaignon@gmail.com


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