selftests: net: support setting recv_size in YNL

recv_size parameter allows constraining the buffer size for dumps.
It's useful in testing kernel handling of dump continuation,
IOW testing dumps which span multiple skbs.

Let the tests set this parameter when initializing the YNL family.
Keep the normal default, we don't want tests to unintentionally
behave very differently than normal code.

Reviewed-by: Joe Damato <jdamato@fastly.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Link: https://patch.msgid.link/20241213152244.3080955-4-kuba@kernel.org


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