afs: Treat vc_error as a CheckCode-translated code

The vcache field vc_error is generally treated as an error code that
has been translated through afs_CheckCode, but this is inconsistent in
a few places. Fix this in a few ways:

 - Adjust afs_nfsrdwr so we do not call afs_CheckCode on vc_error,
   translating the error code twice.

 - Change afs_close to store vc_error in code_checkcode, and have the
   logging code check for specific values in code_checkcode as well.
   Log unknown values of code and code_checkcode, so we can
   distinguish between e.g. a 'code' value of VBUSY, and a
   'code_checkcode' value of ETIMEDOUT.

Reviewed-on: http://gerrit.openafs.org/10634
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: Derrick Brashear <shadow@your-file-system.com>
(cherry picked from commit 34e4a4fed356fbda9fc8ace1d01a080bd09238b0)

Change-Id: Icceee0c82b0704e0d445f96946b493b4be424506
Reviewed-on: http://gerrit.openafs.org/10813
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: D Brashear <shadow@your-file-system.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>