amt: fix size calculation in amt_get_size()

amt: fix size calculation in amt_get_size()

amt_get_size() incorrectly used sizeof(struct iphdr) for the sizes of
IFLA_AMT_DISCOVERY_IP, IFLA_AMT_REMOTE_IP, and IFLA_AMT_LOCAL_IP.
These attributes contain IPv4 addresses (__be32), not full IP headers.

Replace sizeof(struct iphdr) with sizeof(__be32) to avoid over-allocating
netlink message space.

Fixes: b9022b53adad ("amt: add control plane of amt interface")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20260701122329.3562825-1-edumazet@google.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>