btrfs: fix filtering of scratch device in test case 048

btrfs: fix filtering of scratch device in test case 048

The recent commit 4529b20e1aa8f9 ("btrfs/048: amend property validation
cases"), does not properly filter the scratch device because the error
messages are sent to stderr and not to stdout, and the pipe filter only
gets input from the stdout of the btrfs utility. We need to redirect the
stderr of the btrfs utility to its stdout.

Further, the golden output had the path "/mnt/scratch" hardcoded, instead
of using SCRATCH_MNT. Fix that as well.

The test was failing on any setup where the scratch device is not mounted
at "/mnt/scratch".

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>