Tools: hv: vss: fix loop device detection

Commit ea81fdf0 ("Tools: hv: vss: Skip freezing filesystems backed by
loop") added skip for filesystems backed by loop device. However, it seems
the detection of such cases is incomplete.

It was found that with 'devicemapper' storage driver docker creates the
following chain:

NAME					MAJ:MIN
loop0					7:0
..docker-8:4-8473394-pool		253:0
  ..docker-8:4-8473394-eac...		253:1

so when we're looking at the mounted device we see major '253' and not '7'.

Solve the issue by walking /sys/dev/block/*/slaves chain and checking if
there's a loop device somewhere.

Other than that, don't skip mountpoints silently when stat() fails. In case
e.g. SELinux is failing stat we don't want to skip freezing everything
without letting user know about the failure.

Fixes: ea81fdf0 ("Tools: hv: vss: Skip freezing filesystems backed by loop")
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>