generic/590: split XFS RT specific bits out

generic/590: split XFS RT specific bits out

Currently generic/590 runs a very different test on XFS that creates
a lot device and so on.  Split that out into a new XFS-specific test,
and let generic/590 always run using the file system parameter specified
in the config even for XFS.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Anand Jain <asj@kernel.org>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Zorro Lang <zlang@kernel.org>