selinux: selinux_add_opt() callers free memory

The selinux_add_opt() function may need to allocate memory for the
mount options if none has already been allocated, but there is no
need to free that memory on error as the callers handle that.  Drop
the existing kfree() on error to help increase consistency in the
selinux_add_opt() error handling.

This patch also changes selinux_add_opt() to return -EINVAL when
the mount option value, @s, is NULL.  It currently return -ENOMEM.

Link: https://lore.kernel.org/lkml/20220611090550.135674-1-xiujianfeng@huawei.com/T/


Suggested-by: Paul Moore <paul@paul-moore.com>
Signed-off-by: Xiu Jianfeng <xiujianfeng@huawei.com>
[PM: fix subject, rework commit description language]
Signed-off-by: Paul Moore <paul@paul-moore.com>