macvlan: annotate data-races around port->bc_queue_len_used

macvlan: annotate data-races around port->bc_queue_len_used

port->bc_queue_len_used is read and written locklessly,
add READ_ONCE()/WRITE_ONCE() annotations.

While WRITE_ONCE() in macvlan_fill_info() is not yet needed,
it is a prereq for future RTNL avoidance.

Fixes: d4bff72c8401 ("macvlan: Support for high multicast packet rate")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20260401103809.3038139-2-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>