Currently, blkif_queue_request has 2 distinct execution path:
    - Send a discard request
    - Send a read/write request

The function is also allocating grants to use for generating the
request. Although, this is only used for read/write request.

Rather than having a function with 2 distinct execution path, separate
the function in 2. This will also remove one level of tabulation.
Signed-off-by: NJulien Grall <julien.grall@citrix.com>
Reviewed-by: NRoger Pau Monné <roger.pau@citrix.com>
Signed-off-by: NDavid Vrabel <david.vrabel@citrix.com>