xfstests 241: run parallel dbench

xfstests 241: run parallel dbench

ext4 was corrupting inode table bitmaps due to a mishmash of
atomic & nonatomic bitops.  This got broken twice and fixed
twice; let's add a test.

On a 4-way box this reliably ends up with a corrupted filesystem
if we get it wrong (upstream as of 2.6.35 at least is fine again)

Signed-off-by: Eric Sandeen <sandeen@sandeen.net>
Reviewed-by: Dave Chinner <david@fromorbit.com>