ip6mr: Fix RCU list debugging warning

ip6mr_for_each_table() macro uses list_for_each_entry_rcu()
for traversing outside an RCU read side critical section
but under the protection of rtnl_mutex. Hence add the
corresponding lockdep expression to silence the following
false-positive warnings:

[    4.319479] =============================
[    4.319480] WARNING: suspicious RCU usage
[    4.319482] 5.5.4-stable #17 Tainted: G            E
[    4.319483] -----------------------------
[    4.319485] net/ipv6/ip6mr.c:1243 RCU-list traversed in non-reader section!!

[    4.456831] =============================
[    4.456832] WARNING: suspicious RCU usage
[    4.456834] 5.5.4-stable #17 Tainted: G            E
[    4.456835] -----------------------------
[    4.456837] net/ipv6/ip6mr.c:1582 RCU-list traversed in non-reader section!!

Signed-off-by: Amol Grover <frextrite@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>