Fix blockdev mount ordering
Fix blockdev mount ordering

Ensure that the / mount point is always first
but returning an explict value (0) for / mount action.

Signed-off-by: Ryan Harper <7c8ea25f69f5166c51151e84bf5db678c2384828@canonical.com>

CanonicalLtd/subiquity,CanonicalLtd/subiquity