sbitmap: fix sbitmap_for_each_set()

We need to ignore bits in the cleared mask when iterating over all set
bits.

Fixes: ea86ea2c

 ("sbitmap: ammortize cost of clearing bits")
Reported-by: Jens <Axboe@kernel.dk>
Signed-off-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>