io_uring/zcrx: selftests: parse json from ethtool -g

Parse JSON from ethtool -g instead of parsing text output.

Signed-off-by: David Wei <dw@davidwei.uk>
Reviewed-by: Joe Damato <jdamato@fastly.com>
Link: https://patch.msgid.link/20250426195525.1906774-3-dw@davidwei.uk


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