rx: Zero unitialized uio structs

rx: Zero unitialized uio structs

We use some uio structures that were allocated on the stack, but we
only initialize them by initializing individual fields. On some
platforms (Solaris is one known example, but probably not the only
one), there are additional fields we do not initialize. Since we
cannot be certain of what any additional fields there may be, just
zero the whole thing.

This is basically the same change as
I0eae0b49a70aee19f3a9ec118b03cfb3a6bd03a3, but in the rx subtree.

Change-Id: I400144143bb1f47409eccb931daacc8a5058e074
Reviewed-on: http://gerrit.openafs.org/11711
Tested-by: Andrew Deason <adeason@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Perry Ruiter <pruiter@sinenomine.net>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Reviewed-by: Daria Brashear <shadow@your-file-system.com>