Don't run t_cmsg_len on 64-bit architectures; it always fails when
trying to send through the sendit(9) KPI in the kernel due to a size
mismatch between the msghdr and data being sent, that suspiciously
seems like it's related to sizeof pointers instead of scalars, or
something of that ilk

PR: 206543
Sponsored by: EMC / Isilon Storage Division