ipv4: Fix data-races around sysctl_fib_multipath_hash_policy.

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

Fixes: bf4e0a3d

 ("net: ipv4: add support for ECMP hash policy choice")
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: David S. Miller <davem@davemloft.net>