arp: Remove a nest in arp_req_get().

This is a prep patch to make the following changes tidy.

No functional change intended.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://lore.kernel.org/r/20240430015813.71143-5-kuniyu@amazon.com


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