arp: Move ATF_COM setting in arp_req_set().

In arp_req_set(), if ATF_PERM is set in arpreq.arp_flags,
ATF_COM is set automatically.

The flag will be used later for neigh_update() only when
a neighbour entry is found.

Let's set ATF_COM just before calling neigh_update().

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://lore.kernel.org/r/20240430015813.71143-2-kuniyu@amazon.com


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