[IPv6] reassembly: Always compute hash under the fragment lock.

This closes a race where an ipq6hashfn() caller could get a hash value
and race with the cycling of the random seed.  By the time they got to
the read_lock they'd have a stale hash value and might not find
previous fragments of their datagram.

This matches the previous patch to IPv4.

Signed-off-by: Zach Brown <zach.brown@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>