macvlan: fix macvlan_get_size() not reserving space for IFLA_MACVLAN_BC_CUTOFF

macvlan_get_size() does not account for IFLA_MACVLAN_BC_CUTOFF, but
macvlan_fill_info() conditionally includes it when port->bc_cutoff != 1.
This causes nla_put_s32() to fail with -EMSGSIZE when the netlink skb
runs out of space, triggering a WARN_ON in rtnetlink and preventing the
interface from being dumped.

The bug can be reproduced with:

ip link add macvlan0 link eth0 type macvlan mode bridge
ip link set macvlan0 type macvlan bc_cutoff 0
ip -d link show macvlan0 # fails with -EMSGSIZE

The bc_cutoff feature was added in commit 954d1fa1ac93 ("macvlan: Add
netlink attribute for broadcast cutoff"), which added the nla_put_s32()
call in macvlan_fill_info() but missed adding the corresponding
nla_total_size(4) in macvlan_get_size(). A follow-up commit
55cef78c244d ("macvlan: add forgotten nla_policy for
IFLA_MACVLAN_BC_CUTOFF") fixed the missing nla_policy entry but still
did not fix the size calculation.

Fixes: 954d1fa1ac93 ("macvlan: Add netlink attribute for broadcast cutoff")
Signed-off-by: Dudu Lu <phx0fer@gmail.com>
Reviewed-by: Vadim Fedorenko <vadim.fedorenko@linux.dev>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20260413085349.73977-1-phx0fer@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>