selinux: Streamline type determination in security_compute_sid

Simplifies the logic for determining the security context type in
security_compute_sid, enhancing readability and efficiency.

Consolidates default type assignment logic next to type transition
checks, removing redundancy and improving code flow.

Signed-off-by: Canfeng Guo <guocanfeng@uniontech.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>