xfstests: support post-udev device mapper nodes

xfstests: support post-udev device mapper nodes

Because of udevs complaining device mapper now creates /dev/dm-N as the real
device nodes, and just symlinks the /dev/mapper/ names to it.  This would be
easy if everything used the /dev/mapper clear names, but most system utilities
translate them back to the /dev/mapper/ names and thus confuse various test
cases.  Add support to _is_block_dev to read symlinks, and add documentation
on how to run xfstests on device mapper volumes.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Alex Elder <aelder@sgi.com>