ipv6: fix ipv6_prefix_equal64_half mask conversion

Fix the 64bit optimized version of ipv6_prefix_equal to convert the
bitmask to network byte order only after the bit-shift.

The bug was introduced in:

38675170 ipv6: 64bit version of ipv6_prefix_equal().

Signed-off-by: Fabio Baltieri <fabio.baltieri@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>