gensec: Change log level for message when obtaining PAC from gss_get_name_attribute...

gensec: Change log level for message when obtaining PAC from gss_get_name_attribute failed

This is the second part for the issue from commit 8bb4fccd. A KDC that
does not return a PAC first triggers this message, then the "resorting
to local user lookup" one. Change the log level for the "obtaining PAC
via GSSAPI gss_get_name_attribute" message as well to avoid spamming the
logs during normal usage. While changing this message, also remove the
discard_const since it is no longer required.

Signed-off-by: Christof Schmitt <cs@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Autobuild-User(master): Andrew Bartlett <abartlet@samba.org>
Autobuild-Date(master): Wed Jul  6 04:27:03 CEST 2016 on sn-devel-144