afs: Indicate error from afs_osi_Read/Write better

afs: Indicate error from afs_osi_Read/Write better

Currently afs_osi_Read and afs_osi_Write just return -1 on any I/O
error, even though they know the error code given from the OS VFS.
Just return that code instead so the caller can see what the error
was; but negate it, so it's clear that it is an error.

Change-Id: I3d8350da18d075713356137a1cacf182a749fe3e
Reviewed-on: http://gerrit.openafs.org/6412
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>