f2fs: optimize NAT block loading during checkpoint write

f2fs: optimize NAT block loading during checkpoint write

Under stress tests with frequent metadata operations, checkpoint write
time can become excessively long. Analysis shows that the slowdown is
caused by synchronous, one-by-one reads of NAT blocks during checkpoint
processing.

The issue can be reproduced with the following workload:
1. seq 1 650000 | xargs -P 16 -n 1 touch
2. sync # avoid checkpoint write during deleting
3. delete 1 file every 455 files
4. echo 3 > /proc/sys/vm/drop_caches
5. sync # trigger checkpoint write

This patch submits read I/O for all NAT blocks required in the
__flush_nat_entry_set() phase in advance, reducing the overhead of
synchronous waiting for individual NAT block reads.

The NAT block flush latency before and after the change is as below:

|             |NAT blocks accessed|NAT blocks read|Flush time (ms)|
|-------------|-------------------|---------------|---------------|
|Before change|1205               |1191           |158            |
|After change |1264               |1242           |11             |

With a similar number of NAT blocks accessed and read from disk, adding
NAT block readahead reduces the total NAT block flush time by more than
90%.

Signed-off-by: Yongpeng Yang <yangyongpeng@xiaomi.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>