conntrack: Fix 'reverse_nat_packet()' variable datatype.

conntrack: Fix 'reverse_nat_packet()' variable datatype.

The datatype 'pad' in the function 'reverse_nat_packet()' was incorrectly
declared as 'char' instead of 'uint8_t'. This can affect reverse natting
of icmpX packets with padding > 127 bytes.  At the same time, add some
comments regarding 'extract_l3_ipvX' usage in this function.  Found by
inspection.

Fixes: edd1bef468c0 ("dpdk: Add more ICMP Related NAT support.")
Signed-off-by: Darrell Ball <dlu998@gmail.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>