selftests: net: Fix a typo in udpgro_fwd.sh

$rvs -> $rcv

Fixes: a062260a

 ("selftests: net: add UDP GRO forwarding self-tests")
Signed-off-by: Jianguo Wu <wujianguo@chinatelecom.cn>
Link: https://lore.kernel.org/r/d247d7c8-a03a-0abf-3c71-4006a051d133@163.com


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