do_rootfs_setup: fix return bugs

do_rootfs_setup: fix return bugs

Fix return value on bind mount failure.

If we've already mounted the rootfs, exit after the bind mount
rather than re-trying the rootfs mount.  The only case where
this happens is when root is starting a container in a user
namespace and with a block device backing store.

In that case, pre-mount hooks will be executed in the initial
user namespace.  That may be worth fixing.  Or it may be what
we want.  We should think about it and fix it.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>