f2fs: change type for 'sbi->readdir_ra'

Before this patch, the varibale 'readdir_ra' takes effect if it's equal
to '1' or not, so we can change type for it from 'int' to 'bool'.

Signed-off-by: Yuwei Guan <Yuwei.Guan@zeekrlife.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>