ICMP: Make a few cppcheck-inspired fixups.

ICMP: Make a few cppcheck-inspired fixups.

[print-icmp.c:402]: (warning) %d in format string (no. 2) requires 'int'
but the argument type is 'unsigned int'.
[print-icmp.c:560]: (style) The scope of the variable 'sum' can be
reduced.
[print-icmp.c:560]: (style) The scope of the variable 'icmp_sum' can be
reduced.
[print-icmp.c:509]: (warning) %d in format string (no. 1) requires 'int'
but the argument type is 'unsigned int'.
[print-icmp.c:517]: (warning) %d in format string (no. 1) requires 'int'
but the argument type is 'unsigned int'.