f2fs: introduce sysfs readdir_ra to readahead inode block in readdir

This patch introduces a sysfs interface readdir_ra to enable/disable
readaheading inode block in f2fs_readdir. When readdir_ra is enabled,
it improves the performance of "readdir + stat".

For 300,000 files:
	time find /data/test > /dev/null
disable readdir_ra: 1m25.69s real  0m01.94s user  0m50.80s system
enable  readdir_ra: 0m18.55s real  0m00.44s user  0m15.39s system

Signed-off-by: Sheng Yong <[email protected]>
Reviewed-by: Chao Yu <[email protected]>
Signed-off-by: Jaegeuk Kim <[email protected]>
4 files changed