ipv4: igmp: annotate data-races around idev->mr_maxdelay

ipv4: igmp: annotate data-races around idev->mr_maxdelay

[ Upstream commit e4faaf65a75f650ac4366ddff5dabb826029ca5a ]

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

While we are at it, make this field an u32.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://patch.msgid.link/20260122172247.2429403-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>