s3: smbd: Use separate flag to track become_root()/unbecome_root() state.

s3: smbd: Use separate flag to track become_root()/unbecome_root() state.

Early function exit can mean backup_priv is set but we haven't called
become_root(). *Lots* of work by the reviewers went in to checking this
isn't a security issue.

Found by Codenomicon at the Redmond plugfest.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=11339

Signed-off-by: Jeremy Allison <jra@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Volker Lendecke <vl@samba.org>
Autobuild-User(master): Jeremy Allison <jra@samba.org>
Autobuild-Date(master): Thu Jun 25 22:14:58 CEST 2015 on sn-devel-104

(cherry picked from commit e2c4b8967d33b610f9f076c614352e4b8fc7c558)