block/rnbd: replace REQ_OP_FLUSH with REQ_OP_WRITE

block/rnbd: replace REQ_OP_FLUSH with REQ_OP_WRITE

Since flush bios are implemented as writes with no data and
the preflush flag per Christoph's comment [1].

And we need to change it in rnbd accordingly. Otherwise, I
got splatting when create fs from rnbd client.

[  464.028545] ------------[ cut here ]------------
[  464.028553] WARNING: CPU: 0 PID: 65 at block/blk-core.c:751 submit_bio_noacct+0x32c/0x5d0
[ ... ]
[  464.028668] CPU: 0 PID: 65 Comm: kworker/0:1H Tainted: G           OE      6.4.0-rc1 #9
[  464.028671] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.15.0-0-g2dd4b9b-rebuilt.opensuse.org 04/01/2014
[  464.028673] Workqueue: ib-comp-wq ib_cq_poll_work [ib_core]
[  464.028717] RIP: 0010:submit_bio_noacct+0x32c/0x5d0
[  464.028720] Code: 03 0f 85 51 fe ff ff 48 8b 43 18 8b 88 04 03 00 00 85 c9 0f 85 3f fe ff ff e9 be fd ff ff 0f b6 d0 3c 0d 74 26 83 fa 01 74 21 <0f> 0b b8 0a 00 00 00 e9 56 fd ff ff 4c 89 e7 e8 70 a1 03 00 84 c0
[  464.028722] RSP: 0018:ffffaf3680b57c68 EFLAGS: 00010202
[  464.028724] RAX: 0000000000060802 RBX: ffffa09dcc18bf00 RCX: 0000000000000000
[  464.028726] RDX: 0000000000000002 RSI: 0000000000000000 RDI: ffffa09dde081d00
[  464.028727] RBP: ffffaf3680b57c98 R08: ffffa09dde081d00 R09: ffffa09e38327200
[  464.028729] R10: 0000000000000000 R11: 0000000000000000 R12: ffffa09dde081d00
[  464.028730] R13: ffffa09dcb06e1e8 R14: 0000000000000000 R15: 0000000000200000
[  464.028733] FS:  0000000000000000(0000) GS:ffffa09e3bc00000(0000) knlGS:0000000000000000
[  464.028735] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  464.028736] CR2: 000055a4e8206c40 CR3: 0000000119f06000 CR4: 00000000003506f0
[  464.028738] Call Trace:
[  464.028740]  <TASK>
[  464.028746]  submit_bio+0x1b/0x80
[  464.028748]  rnbd_srv_rdma_ev+0x50d/0x10c0 [rnbd_server]
[  464.028754]  ? percpu_ref_get_many.constprop.0+0x55/0x140 [rtrs_server]
[  464.028760]  ? __this_cpu_preempt_check+0x13/0x20
[  464.028769]  process_io_req+0x1dc/0x450 [rtrs_server]
[  464.028775]  rtrs_srv_inv_rkey_done+0x67/0xb0 [rtrs_server]
[  464.028780]  __ib_process_cq+0xbc/0x1f0 [ib_core]
[  464.028793]  ib_cq_poll_work+0x2b/0xa0 [ib_core]
[  464.028804]  process_one_work+0x2a9/0x580

[1]. https://lore.kernel.org/all/ZFHgefWofVt24tRl@infradead.org/

Signed-off-by: Guoqing Jiang <guoqing.jiang@linux.dev>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Link: https://lore.kernel.org/r/20230512034631.28686-1-guoqing.jiang@linux.dev
Signed-off-by: Jens Axboe <axboe@kernel.dk>