It is already using atomic test_and_set_bit to do the
allocation.

There is some microscopic chance of starvation, but it is
so microscopic that it should never happen in reality.
Signed-off-by: NDon Brace <don.brace@pmcs.com>
Reviewed-by: NWebb Scales <webbnh@hp.com>
Signed-off-by: NChristoph Hellwig <hch@lst.de>