btrfs: make a btrfs version of writeback error reporting test

btrfs: make a btrfs version of writeback error reporting test

For btrfs, we can test how it reports data writeback errors on fsync by
implementing a suggestion from Chris Mason:

Build a filesystem with 2 devices that stripes the data across
both devices, but mirrors metadata across both. Then, make one
of the devices fail and test what it does.

[eguan: add comments about creating btrfs with "-d raid0 -m raid1"]

Cc: Chris Mason <clm@fb.com>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>