ICMP: Replace a 'struct in_addr' member type with a 'nd_ipv4' one

ICMP: Replace a 'struct in_addr' member type with a 'nd_ipv4' one

In 'struct icmp'.
This avoid some 'undefined-behavior' warnings with clang and option
-fsanitize=undefined enabled.

Moreover:
Remove unneeded '&' when getting a pointer to a nd_ipv4 type.
Fix indentation.

The warnings were like:
print-icmp.c:355:2: runtime error: member access within misaligned address
0x61d00001eabe for type 'const struct icmp', which requires 4 byte alignment
0x61d00001eabe: note: pointer points here
 ef 01 02 03 08 00  90 e1 00 03 00 00 00 00  00 00 00 05 ed 60 ab cd  ab cd ab cd ab cd ab cd  ab cd
             ^
SUMMARY: AddressSanitizer: undefined-behavior print-icmp.c:355:2 in