zfs: Fix on-disk data inconsistency on shutdown

This problem was found when running 'tests/tst-zfs-mount.so' multiple times.
At the first time, all tests succeed, however, a subsequent run would
fail at the test: 'mkdir /foo/bar', the error message reported
that the target file already exists.

The test basically creates a directory /foo/bar, rename it to /foo/bar2,
then remove /foo/bar2. How could /foo/bar still be there?

Quite simple. Our shutdown function calls unmount_rootfs() which will
attempt to unmount zfs with the flag MNT_FOURCE, however, it's not being
passed to zfs_unmount(), neither unmount_rootfs() tests itself the
return status (which was always getting failures previously).
So OSv is really being shutdown while there is remaining data waiting to
be synced with the backing store. As a result, inconsitency.

This problem was fixed by passing the flag to VFS_UNMOUNT which will now
unmount the fs properly on sudden shutdowns.

Signed-off-by: Raphael S. Carvalho <raphaelsc@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>