generic/038: speed up file creation

generic/038: speed up file creation

Now that generic/038 is running on my test machine, I notice how
slow it is:

generic/038      692s

11-12 minutes for a single test is way too long.
The test is creating
400,000 single block files, which can be easily parallelised and
hence run much faster than the test is currently doing.

Split the file creation up into 4 threads that create 100,000 files
each. 4 is chosen because XFS defaults to 4AGs, ext4 still has decent
speedups at 4 concurrent creates, and other filesystems aren't hurt
by excessive concurrency. The result:

generic/038      237s

on the same machine, which is roughly 3x faster and so it (just)
fast enough to to be considered acceptible.

[Eryu Guan: reduced number of files to minimum needed to reproduce
 btrfs problem reliably, added $LOAD_FACTOR scaling for longer
 running.]

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>