selftests: drv-net: Strip '@' prefix from bpftrace map keys

The '@' prefix in bpftrace map keys is specific to bpftrace and can be
safely removed when processing results. This patch modifies the bpftrace
utility to strip the '@' from map keys before storing them in the result
dictionary, making the keys more consistent with Python conventions.

Signed-off-by: Breno Leitao <leitao@debian.org>
Link: https://patch.msgid.link/20250714-netpoll_test-v7-2-c0220cfaa63e@debian.org


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