As standard mount.Unmount does what we need, let's use it.
In addition, this adds ignoring "not mounted" condition, which
was previously implemented (see PR#33329, commit cfa2591d3f26)
via a very expensive call to mount.Mounted().
Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>
(cherry picked from commit 77bc327e24a60791fe7e87980faf704cf7273cf9)
Upstream-commit: 893b24b80db170279d5c9532ed508a81c328de5e
Component: engine