namei: tweak getlinkbyte error handling

namei: tweak getlinkbyte error handling

in the event of a pread error, add the same error exit path
trigger that we have elsewhere

Change-Id: Id7a371f3fd710f3c6dad21bccb0662cf8562d99f
Reviewed-on: http://gerrit.openafs.org/5843
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Reviewed-by: Jeffrey Altman <jaltman@secure-endpoints.com>
Tested-by: Jeffrey Altman <jaltman@secure-endpoints.com>