genhd: fix leftover might_sleep() in blk_free_devt()

Commit 2da78092 changed the locking from a mutex to a spinlock,
so we now longer sleep in this context. But there was a leftover
might_sleep() in there, which now triggers since we do the final
free from an RCU callback. Get rid of it.

Reported-by: Pontus Fuchs <pontus.fuchs@gmail.com>
Signed-off-by: Jens Axboe <axboe@fb.com>