SELinux: move security_skb_extlbl_sid() out of the security server

As suggested, move the security_skb_extlbl_sid() function out of the security
server and into the SELinux hooks file.

Signed-off-by: Paul Moore <paul.moore@hp.com>
Acked-by: Stephen Smalley <sds@tycho.nsa.gov>
Signed-off-by: James Morris <jmorris@namei.org>