ipv6: fix typo in fib6_net_exit()

[ Upstream commit 32a805ba ]

IPv6 FIB should use FIB6_TABLE_HASHSZ, not FIB_TABLE_HASHSZ.

Fixes: ba1cc08d ("ipv6: fix memory leak with multiple tables during netns destruction")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>