ipmr: Use MAXVIFS in mroute_msgsize().

ipmr: Use MAXVIFS in mroute_msgsize().

mroute_msgsize() calculates skb size needed for ipmr_fill_mroute().

The size differs based on mrt->maxvif.

We will drop RTNL for ipmr_rtm_getroute() and mrt->maxvif may
change under RCU.

To avoid -EMSGSIZE, let's calculate the size with the maximum
value of mrt->maxvif, MAXVIFS.

struct rtnexthop is 8 bytes and MAXVIFS is 32, so the maximum delta
is 256 bytes, which is small enough.

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20260228221800.1082070-5-kuniyu@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>