ila: cleanup checksum diff

Consolidate computing checksum diff into one function.

Add get_csum_diff_iaddr that computes the checksum diff between
an address argument and locator being written. get_csum_diff
calls this using the destination address in the IP header as
the argument.

Also moved ila_init_saved_csum to be close to the checksum

diff functions.

Signed-off-by: Tom Herbert <tom@quantonium.net>
Signed-off-by: David S. Miller <davem@davemloft.net>