staging: erofs: surround fault_injection ralted option parsing using CONFIG_EROFS_FAULT_INJECTION

It's a little bit strange when fault_injection related
option fail with -EINVAL which was already disabled
from config, so surround all fault_injection related option
parsing code using CONFIG_EROFS_FAULT_INJECTION. Meanwhile,
slightly change warning message to keep consistency with
option POSIX_ACL and FS_XATTR.

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