ipv6: Fix sysctl max for fib_multipath_hash_policy

Cited commit added the possible value of '2', but it cannot be set. Fix
it by adjusting the maximum value to '2'. This is consistent with the
corresponding IPv4 sysctl.

Before:

# sysctl -w net.ipv6.fib_multipath_hash_policy=2
sysctl: setting key "net.ipv6.fib_multipath_hash_policy": Invalid argument
net.ipv6.fib_multipath_hash_policy = 2
# sysctl net.ipv6.fib_multipath_hash_policy
net.ipv6.fib_multipath_hash_policy = 0

After:

# sysctl -w net.ipv6.fib_multipath_hash_policy=2
net.ipv6.fib_multipath_hash_policy = 2
# sysctl net.ipv6.fib_multipath_hash_policy
net.ipv6.fib_multipath_hash_policy = 2

Fixes: d8f74f09

 ("ipv6: Support multipath hashing on inner IP pkts")
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Stephen Suryaputra <ssuryaextr@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>