btrfs/143: make test case more reliable

btrfs/143: make test case more reliable

This changes to use '_scratch_cycle_mount' to drop all caches btrfs
could have in order to avoid an issue that drop_caches somehow
doesn't work on Nikolay's box.

Also use bash -c to run 'read' only when %pid is odd so that we can
read the faulty disk.

Reported-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Tested-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>