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.

Reviewed-on: http://gerrit.openafs.org/8369
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
(cherry picked from commit 48b60478897e7cb22e9cc1adfa3b71c188572093)

Change-Id: I32a079934c8d3ecd3d7316cfc7eb8da01580a08e
Reviewed-on: http://gerrit.openafs.org/9286
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>
Tested-by: BuildBot <buildbot@rampaginggeek.com>