test.py: extend failure patterns

This is a temporary fix. I really feel we need failure detection based
on exit code.

Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
vfs: MNT_FORCE unmount

Add sys_umount2() and implement support for MNT_FORCE that will be used
to force rootfs unmount at poweroff.

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
vfs: Fix sys_pivot_root()

Make sure the new root vnode is not evicted in vflush() and that we
don't keep reference to the old root dentry in ->m_covered.

Fixes negative dentry count when pivoted root is unmounted.

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
loader: Unmount rootfs at poweroff

Make sure ZFS intention log is flushed to disk by unmounting rootfs at
poweroff.

Fixes #108.

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>