block: collapse blk_alloc_request() into get_request()

Allocation failure handling in get_request() is about to be updated.
To ease the update, collapse blk_alloc_request() into get_request().

This patch doesn't introduce any functional change.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>