selftests: net: avoid memory leak

The buffer be used without free,fix it to avoid memory leak.

Signed-off-by: Zongmin Zhou <zhouzongmin@kylinos.cn>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250901054557.32811-1-min_halo@163.com


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