bcachefs: add REQ_SYNC and REQ_IDLE in write dio

when writing file with direct_IO on bcachefs, then performance is
much lower than other fs due to write back throttle in block layer:

        wbt_wait+1
        __rq_qos_throttle+32
        blk_mq_submit_bio+394
        submit_bio_noacct_nocheck+649
        bch2_submit_wbio_replicas+538
        __bch2_write+2539
        bch2_direct_write+1663
        bch2_write_iter+318
        aio_write+355
        io_submit_one+1224
        __x64_sys_io_submit+169
        do_syscall_64+134
        entry_SYSCALL_64_after_hwframe+110

add set REQ_SYNC and REQ_IDLE in bio->bi_opf as standard dirct-io
Signed-off-by: zhuxiaohui <zhuxiaohui.400@bytedance.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/fs-io-direct.c

@@ -536,7 +536,7 @@ static __always_inline long bch2_dio_write_loop(struct dio_write *dio)
 		if (likely(!dio->iter.count) || dio->op.error)
 			break;
 
-		bio_reset(bio, NULL, REQ_OP_WRITE);
+		bio_reset(bio, NULL, REQ_OP_WRITE | REQ_SYNC | REQ_IDLE);
 	}
 out:
 	return bch2_dio_write_done(dio);
@@ -618,7 +618,7 @@ ssize_t bch2_direct_write(struct kiocb *req, struct iov_iter *iter)
 
 	bio = bio_alloc_bioset(NULL,
 			       bio_iov_vecs_to_alloc(iter, BIO_MAX_VECS),
-			       REQ_OP_WRITE,
+			       REQ_OP_WRITE | REQ_SYNC | REQ_IDLE,
 			       GFP_KERNEL,
 			       &c->dio_write_bioset);
 	dio = container_of(bio, struct dio_write, op.wbio.bio);