selftests: net: cleanup busy_poller.c

Fix various integer type conversions by using strtoull and a temporary
variable which is bounds checked before being casted into the
appropriate cfg_* variable for use by the test program.

While here:
  - free the strdup'd cfg string for overall hygenie.
  - initialize napi_id = 0 in setup_queue to avoid warnings on some
    compilers.

Signed-off-by: Joe Damato <jdamato@fastly.com>
Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://patch.msgid.link/20241204163239.294123-1-jdamato@fastly.com


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