xen, blkfront: factor out flush-related checks from do_blkif_request()

This commit factors out some checks related to the request insertion
path, which can be done in an function instead of by itself.

Reviewed-by: David Vrabel <david.vrabel@citrix.com>
Signed-off-by: Arianna Avanzini <avanzini.arianna@gmail.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

diff --cc drivers/block/xen-blkfront.c
Simple merge