generic/204: use fixed log size for XFS

generic/204: use fixed log size for XFS

In changing the default log sizes in mkfs, the freespace
calculations in generic/204 are no longer valid and so it fails with
ENOSPC before ti has finished creating the necessary files.. Make
the test use a fixed log size of 5MB for XFS so that freespace
calculations remain valid and the test passes regardless of whether
we have a new or old mkfs binary.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Rich Johnston <rjohnston@sgi.com>