linux: zvols: correctly detect flush requests

since 4.10, bio->bi_opf needs to be checked to determine all kinds of
flush requests. this was the case prior to the commit referenced below,
but the order of ifdefs was not the usual one (newest up top), which
might have caused this to slip through.

this fixes a regression when using zvols as Qemu block devices, but
might have broken other use cases as well. the symptoms are that all
sync writes from within a VM configured to use such a virtual block
devices are ignored and treated as async writes by the host ZFS layer.

this can be verified using fio in sync mode inside the VM, for example
with

fio \
--filename=/dev/sda --ioengine=libaio --loops=1 --size=10G \
--time_based --runtime=60 --group_reporting --stonewall --name=cc1 \
--description="CC1" --rw=write --bs=4k --direct=1 --iodepth=1 \
--numjobs=1 --sync=1

which shows an IOPS number way above what the physical device underneath
supports, with "zpool iostat -r 1" on the hypervisor side showing no
sync IO occuring during the benchmark.

with the regression fixed, both fio inside the VM and the IO stats on
the host show the expected numbers.

Fixes: 846b5985192467acabf5484ae610b4b37b7f8162
"config: remove HAVE_REQ_OP_* and HAVE_REQ_*"

Signed-off-by: Fabian-Gruenbichler <f.gruenbichler@proxmox.com>
Co-authored-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Closes #17131
(cherry picked from commit 41823a0ede88377c39ad218ddd1e60776ea5f466)