f2fs: set readdir_ra by default

It gives general readdir improvement.
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/super.c

@@ -1297,6 +1297,7 @@ static void default_options(struct f2fs_sb_info *sbi)
 	sbi->inline_xattr_size = DEFAULT_INLINE_XATTR_ADDRS;
 	sbi->whint_mode = WHINT_MODE_OFF;
 	sbi->alloc_mode = ALLOC_MODE_DEFAULT;
+	sbi->readdir_ra = 1;
 
 	set_opt(sbi, BG_GC);
 	set_opt(sbi, INLINE_XATTR);