bcachefs: add REQ_SYNC and REQ_IDLE in write dio

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>