Move local union such that it doesn't escape (and get optimized out).

Bug: 23239997

The Clang update exposed a latent bug in the code here, where a pointer
to a local variable escaped the encapsulating block. Clang noticed the
end of this object's lifetime, and removed assignments to its original
storage (because they are now dead assignments). By moving the union out
of the block, it will survive until the sendmsg() call, and the expected
writes will be restored.

Change-Id: If2106d2f53d761ddca6dd26ab2648244d737dcd9