afs: Avoid -1 error for vreadUIO/vwriteUIO

afs: Avoid -1 error for vreadUIO/vwriteUIO

Commit c6b61a45 (afs: Verify osi_UFSOpen worked) added various checks
to return an error if a given osi_UFSOpen failed. However, two of
these checks (in afs_UFSReadUIO and afs_UFSWriteUIO) result in us
returning -1 on error, in functions that otherwise return errno codes
(e.g. ENOSPC). An error code of -1 might get interpreted as
RX_CALL_DEAD, which would be rather confusing, so use EIO as a generic
error instead.

Reviewed-on: https://gerrit.openafs.org/13931
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
(cherry picked from commit 360b9d5d71fb1de142ae4efd4660732476855a3f)

Change-Id: I4c6773affe02cc7a3ca01cf25bea21c960d98e87
Reviewed-on: https://gerrit.openafs.org/13938
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Cheyenne Wills <cwills@sinenomine.net>
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Reviewed-by: Marcio Brito Barbosa <mbarbosa@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>