xfs/841: create a block device that must exist

xfs/841: create a block device that must exist

This test currently creates a block device node for /dev/ram0,
which isn't guaranteed to exist, and can thus cause the test to
fail with:

mkfs.xfs: cannot open $TEST_DIR/proto/blockdev: No such device or address

Instead, create a node for the backing device for $TEST_DIR, which must
exist.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Reviewed-by: Luca Di Maio <luca.dimaio1@gmail.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>