ipv4: Fix data-races around sysctl_fib_multipath_hash_fields.

While reading sysctl_fib_multipath_hash_fields, it can be changed
concurrently.  Thus, we need to add READ_ONCE() to its readers.

Fixes: ce5c9c20

 ("ipv4: Add a sysctl to control multipath hash fields")
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>