xfs/104: use fixed log size

xfs/104: use fixed log size

In changing the default log sizes in mkfs, the freespace
calculations in xfs/104 are no longer valid and so it fails with
ENOSPC before running any of the growfs tests. Make the test use a
fixed log size of 5MB 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>