auth: eliminate pointless retries in ktc_ListTokensEx

auth: eliminate pointless retries in ktc_ListTokensEx

ktc_ListTokensEx is an iterator to provide the names of each cell for
which a user has a token set.  It does this by looking for the 1 through
nth token set for a given user.  However, as currently implemented,
it always continues searching up to the 100x safety limit even when
there are no more token sets for the user.

Instead, return immediately when VIOC_GETTOK2 returns EDOM (no more
tokens for this user).

Introduced by commit a86ad262d2a8be36f43ab0885a84dde37ddfc464 'auth: Add
the ktc_ListTokensEx function'.

Reviewed-on: https://gerrit.openafs.org/13598
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
(cherry picked from commit 95ae30c30d98a3219fd021e0ed83200c1b6c266f)

Change-Id: Ib0489b78b92861ae25b625b3054caf68f3b0a074
Reviewed-on: https://gerrit.openafs.org/13678
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>