sbitmap: simplify wrap check

__sbitmap_get_word() doesn't warp if it's starting from the beginning
(i.e. initial hint is 0). Instead of stashing the original hint just set
@wrap accordingly.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>