Fix buffer lengths.

After r319369, the RPC code validates caller supplied buffer length in
taddr2uaddr. When no -h is specified, the sizeof(ai_addr) is used,
which is always smaller than the required size and therefore uaddr
would be NULL, causing the kernel to copyin() from userland NULL
and fail with EFAULT.

Reviewed by: kevlo (via Telegram)
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D11151