afs: afs_osi_Read/Write returns negative on error

afs: afs_osi_Read/Write returns negative on error

afs_osi_Read and afs_osi_Write need to return negative values on
error. EIO is not negative; return -EIO so we don't accidentally
return "success" if someone requested to read or write EIO bytes.

Reviewed-on: http://gerrit.openafs.org/6411
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>
(cherry picked from commit 456ee898ae77b201275363f4efc0098c28e195d5)

Change-Id: Idd2e6b85d27a914cac0169a6e8fdee49ff262dc0
Reviewed-on: http://gerrit.openafs.org/7937
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>