arp: do not assume dev_hard_header() does not change skb->head

arp: do not assume dev_hard_header() does not change skb->head

arp_create() is the only dev_hard_header() caller
making assumption about skb->head being unchanged.

A recent commit broke this assumption.

Initialize @arp pointer after dev_hard_header() call.

Fixes: db5b4e39c4e6 ("ip6_gre: make ip6gre_header() robust")
Reported-by: syzbot+58b44a770a1585795351@syzkaller.appspotmail.com
Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20260107212250.384552-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>