rx: Handle negative returns on packet reads

rxi_RecvMsg returns an int, because it can return a negative value upon
error. Don't store its return value as an unsigned int, because this may
hide the potential errors.

Modify the error handling loop so that errors get to where they are
intended.

Reviewed-on: http://gerrit.openafs.org/7087
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>
(cherry picked from commit 438d6ba63cda7d7484c545a4fd181803e84d68eb)

Change-Id: I4f479a4e722b5f298ba2be4e86816ebddede701d
Reviewed-on: http://gerrit.openafs.org/10733
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>