mkarchroot: do not create /etc/mtab

As of filesystem-2011.12 this is not needed anylonger, and indeed blocks the
chroot from being upgraded. As a workaround, recreating the chroot fixes the
problem.

Fixes FS#27640.

Reported-by: Andrea Scarpino <andrea@archlinux.org>
Signed-off-by: Tom Gundersen <teg@jklm.no>
Signed-off-by: Pierre Schmitz <pierre@archlinux.de>