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: Don Brace <don.brace@pmcs.com>
Reviewed-by: Webb Scales <webbnh@hp.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>