staging: erofs: code cleanup for erofs_show_options()

Add new helper erofs_get_fault_rate() to get fault rate instead of
directly getting it from sbi, so we can remove the macro check
surrounding it.

Signed-off-by: Chengguang Xu <cgxu519@gmx.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Reviewed-by: Gao Xiang <gaoxiang25@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>