block-backend: Reorder flush/pdiscard function definitions

Move all variants of the flush/pdiscard functions to a single place and
put the blk_co_*() version first because it is called by all other
variants (and will become static in the next patch).

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20200407121259.21350-2-kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>