block: Remove NULL check in bdrv_co_flush

Reported by Coverity. We already use bs in bdrv_inc_in_flight before
checking for NULL. It is unnecessary as all callers pass non-NULL bs, so
drop it.

Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>