attr: nla_get_u64() should return 0 if the attribute does not fully contain 64 bit

Manually "inline" nla_memcpy() to nla_get_u64() and change the behavior
to return always zero (0) if the attribute does not contain at least
sizeof(uint64_t) bytes. Considering endianness, reading a truncated integer
does not seem to be useful and should result in a defined behavior
instead.

Acked-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: Thomas Haller <thaller@redhat.com>