Clang 4.0 complains:

../tests/test-hash.c:160:16: error: taking address of packed member 'b' of
class or structure 'offset_ovs_u128' may result in an unaligned pointer value
      [-Werror,-Waddress-of-packed-member]
        in0 = &in0_data.b;

Set the bit in the aligned u128 first then copy the contents into the
offset u128 so that we don't have to take the address of the non-aligned
u128 and pass it to set_bit128.

For the 256byte_hash, fix it up so that it's actually testing the 256B
hash inside a 32-bit offset u128 as well.

Suggested-by: Ben Pfaff <blp@ovn.org>
Signed-off-by: Joe Stringer <joe@ovn.org>
Acked-by: Ben Pfaff <blp@ovn.org>