ipv6: fix incorrect bitwise operator used on rt6i_flags

The use of the | operator always leads to true which looks rather
suspect to me. Fix this by using & instead to just check the
RTF_CACHE entry bit.

Detected by CoverityScan, CID#1457734, #1457747 ("Wrong operator used")

Fixes: 35732d01 ("ipv6: introduce a hash table to store dst cache")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Wei Wang <weiwan@google.com>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: David S. Miller <davem@davemloft.net>