gethtbyaddr: Reserve space for mapping IPv4 address to IPv6 address

Currently function return address space size is reserved by the address family.
If Netd does reverse DNS query with synthesized IPv6 address failed, Netd
does fallback to try IPv4 DNS query without NAT64 prefix. When Netd receives
answers, hostname and IPv4 address, from hostent structure function returned,
Netd maps the return address from IPv4 to IPv6 in the place. Reserving enough
address space for either IPv4 or IPv6 avoids buffer overflow while doing
address family remapping in Netd.

Bug: 78545619
Test: netd_{unit,integration}_test pass
Change-Id: I69035d1c81b70058d9240c2bad02b80a9883bce3
2 files changed