AFS: Correctly translate auth error aborts and don't failover in such cases

Authentication error abort codes should be translated to appropriate
Linux error codes, rather than all being translated to EREMOTEIO - which
indicates that the server had internal problems.

Additionally, a server shouldn't be marked unavailable and the next
server tried if an authentication error occurs.  This will quickly make
all the servers unavailable to the client.  Instead the error should be
returned straight to the user.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>