RT6: Replace 'struct in6_addr' members type with a 'nd_ipv6' one

RT6: Replace 'struct in6_addr' members type with a 'nd_ipv6' one

In 'struct ip6_hdr', 'struct ip6_rthdr0' and 'struct ip6_srh'.
This avoid some 'undefined-behavior' warnings with clang and option
-fsanitize=undefined enabled.

print-rt6.c:66:3: runtime error: member access within misaligned address
0x61d00001eab6 for type 'const struct ip6_rthdr0', which requires 4 byte
alignment
0x61d00001eab6: note: pointer points here
 00 00 00 04 3a 02  00 01 00 00 00 00 22 00  00 00 00 00 02 10 00 02  00 00 00 00 00 04 80 00  d3 ab
             ^
SUMMARY: AddressSanitizer: undefined-behavior print-rt6.c:66:3 in