f2fs: cache: wake up f2fs_writeback when exceeding threshold

This patch introduces cache_wb_dirty_threshold and cache_wb_total_threshold
sysfs nodes under /sys/fs/f2fs/<disk>/, and wakes up the f2fs_writeback
kthread when dirty meta/node caches exceed cache_wb_dirty_threshold and
total cached entries exceed cache_wb_total_threshold; a zero threshold
disables the corresponding check. By default, cache_wb_dirty_threshold is
8192 and cache_wb_total_threshold is 0.

If both thresholds are zero, f2fs_writeback only runs periodically, unless
checkpoint is disabled, as nothing else will flush dirty caches then.

f2fs_writeback won't be woken up unless there are enough dirty caches for
f2fs_write_{meta,node}_caches() to gather. And if thresholds are exceeded
but one round fails to write any caches, e.g. due to cp_error, freezing,
frozen fs, or lock contention, threshold based wakeup is throttled until
a later round makes progress, in order to avoid busy-looping. To detect
progress, introduce a new parameter for f2fs_write_{meta,node}_caches()
and f2fs_writeback_node_caches() to return the number of caches submitted
for writeback.

Thanks Daeho and Wenjie for help and suggestion on this patch.

Suggested-by: Daeho Jeong <daehojeong@google.com>
Suggested-by: Wenjie Qi <qiwenjie@xiaomi.com>
Signed-off-by: Chao Yu <chao@kernel.org>
Reviewed-by: Wenjie Qi <qiwenjie@xiaomi.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

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