The algorithm for checksum neutral mapping is incorrect. This problem
was being hidden since we were previously always performing checksum
offload on the translated addresses and only with IPv6 HW csum.
Enabling an ILA router shows the issue.

Corrected algorithm:

old_loc is the original locator in the packet, new_loc is the value
to overwrite with and is found in the lookup table. old_flag is
the old flag value (zero of CSUM_NEUTRAL_FLAG) and new_flag is
then (old_flag ^ CSUM_NEUTRAL_FLAG) & CSUM_NEUTRAL_FLAG.

Need SUM(new_id + new_flag + diff) == SUM(old_id + old_flag) for
checksum neutral translation.

Solving for diff gives:

diff = (old_id - new_id) + (old_flag - new_flag)

compute_csum_diff8(new_id, old_id) gives old_id - new_id

If old_flag is set
   old_flag - new_flag = old_flag = CSUM_NEUTRAL_FLAG
Else
   old_flag - new_flag = -new_flag = ~CSUM_NEUTRAL_FLAG

Tested:
  - Implemented a user space program that creates random addresses
    and random locators to overwrite. Compares the checksum over
    the address before and after translation (must always be equal)
  - Enabled ILA router and showed proper operation.
Signed-off-by: Tom Herbert <tom@herbertland.com>
Signed-off-by: David S. Miller <davem@davemloft.net>