dfs: always call create_conn_struct with root privileges

This fixes a bug in dfs_samba4 identified by Daniel Müller.

create_conn_struct calls SMB_VFS_CONNECT which requires root privileges.
SMB_VFS_CONNECT in turn calls dfs_samba4_connect which connects to samdb.

Calls were made to this function without ever becoming root (notably via setup_dfs_referral)
which resulted in an error and the VFS connect failing. This happens when you have an active
directory domain controller with host msdfs = yes in smb.conf and dfs links in place.

Signed-off-by: Garming Sam <garming@catalyst.net.nz>
Reviewed-by: Bjoern Baumbach <bb@sernet.de>
Reviewed-by: Jeremy Allison <jra@samba.org>

Autobuild-User(master): Jeremy Allison <jra@samba.org>
Autobuild-Date(master): Fri Jan 10 20:11:03 CET 2014 on sn-devel-104