block: ensure that struct blk_mq_alloc_data is fully initialized

On x86, rep stos will be emitted to clear the the blk_mq_alloc_data
struct, as not all members are being explicitly initialied. Depending on
the type of CPU, this is a noticeable slowdown compared to just ensuring
that the struct is fully initialized when setup.

For the 4 spots that setup a struct blk_mq_alloc_data on the stack,
ensure all members are being initialized.

Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>