f2fs: relocate readdir_ra configure initialization

readdir_ra is sysfs configuration instead of mount option, so it should
not be initialized in default_options(), otherwise after remount, it can
be reset to be enabled which may not as user wish, so let's move it to
f2fs_tuning_parameters().

Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>