ipv6: Protect fib6_link_table() with spinlock.

ipv6: Protect fib6_link_table() with spinlock.

We will get rid of RTNL from RTM_NEWROUTE and SIOCADDRT.

If the request specifies a new table ID, fib6_new_table() is
called to create a new routing table.

Two concurrent requests could specify the same table ID, so we
need a lock to protect net->ipv6.fib_table_hash[h].

Let's add a spinlock to protect the hash bucket linkage.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Link: https://patch.msgid.link/20250418000443.43734-13-kuniyu@amazon.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>