block: skip redundant flush for O_DSYNC direct writes - #1130

For an O_DIRECT | O_DSYNC write to a block device, dio_bio_write_op()
adds REQ_FUA to the bio, so the data is durable when the direct-IO
returns.  blkdev_write_iter() then falls through to
generic_write_sync(), which for O_DSYNC turns into an additional
REQ_PREFLUSH.  That flush is redundant: the FUA write already made
the data persistent.

Skip generic_write_sync() when the direct-IO path already provided
durability via FUA.  The buffered and direct_write_fallback() paths
are unchanged.

Measured on a Seagate ST20000NM007D (20 TB, 7200 rpm, fua=1,
write_cache=write back), Linux v7.2.0-rc7, single-threaded pwrite()
loop opening the raw block device with O_WRONLY | O_DIRECT | O_DSYNC,
4 KiB writes for 60 s:

  Sequential 4 KiB writes:
                            baseline    patched
    IOPS                       119.7     7497.0
    avg latency (us)            8357        133
    p50 latency (us)            8346        127
    p99 latency (us)            8368        395
    p99.9 latency (us)          8728        569

  Random 4 KiB writes (100 GiB span):
                            baseline    patched
    IOPS                       156.1      666.4
    avg latency (us)            6405       1500
    p50 latency (us)            6186       1450
    p99 latency (us)           16133       2285
    p99.9 latency (us)         17250       9916

Signed-off-by: Zhenxian Ma <mzx199711@gmail.com>
Signed-off-by: Zhenxian Ma <mazhenxian@xiaohongshu.com>