selinux: use sk blob accessor in socket permission helpers

In the Linux kernel, the following vulnerability has been resolved:

selinux: use sk blob accessor in socket permission helpers

SELinux socket state lives in the composite LSM socket blob.

sock_has_perm() and nlmsg_sock_has_extended_perms() currently

dereference sk->sk_security directly, which assumes the SELinux socket

blob is at offset zero.

In stacked configurations that assumption does not hold. If another LSM

allocates socket blob storage before SELinux, these helpers may read the

wrong blob and feed invalid SID and class values into AVC checks.

Use selinux_sock() instead of accessing sk->sk_security directly.