common.sh: Change umount function to account for already deleted mounts.

BUG=chromium-os:31250
TEST=enter chroot; kill ssh-agent; exit chroot

Change-Id: Iea8e0fdd9a82047bb756a459dffc2ef59615074c
Reviewed-on: https://gerrit.chromium.org/gerrit/31929
Tested-by: Zdenek Behan <zbehan@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Commit-Ready: Zdenek Behan <zbehan@chromium.org>
1 file changed