generic: use mount point instead of device name

generic: use mount point instead of device name

A tmpfs mount does not involve any block device, its $SCRATCH_DEV is
nothing but a place-holder, so apply 'df' or 'stat' to its mount point
$SCRATCH_MNT instead of to $SCRATCH_DEV.

Signed-off-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Junho Ryu <jayr@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dave Chinner <david@fromorbit.com>