bfq: Fix computation of shallow depth

BFQ computes number of tags it allows to be allocated for each request type
based on tag bitmap. However it uses 1 << bitmap.shift as number of
available tags which is wrong. 'shift' is just an internal bitmap value
containing logarithm of how many bits bitmap uses in each bitmap word.
Thus number of tags allowed for some request types can be far to low.
Use proper bitmap.depth which has the number of tags instead.

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jens Axboe <axboe@kernel.dk>