afs: Make last_error always useful

afs: Make last_error always useful

Currently we record last_error as the last getuerror() we got when
failing to read in a slot in UFSGetDSlot. For kernels that do not have
getuerror(), this variable is currently useless, and we do not record
anywhere what the last error received was (besides logging it via
afs_warn).

So, for non-uerror, just record what 'code' we got, so we at least
have something.

Change-Id: Iede39bbeee48ea48f380b788f2968886a831a918
Reviewed-on: http://gerrit.openafs.org/8369
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>