SELinux: null-terminate context string in selinux_xfrm_sec_ctx_alloc

xfrm_audit_log() expects the context string to be null-terminated
which currently doesn't happen with user-supplied contexts.

Signed-off-by: Venkat Yekkirala <vyekkirala@TrustedCS.com>
Acked-by: Stephen Smalley <sds@tycho.nsa.gov>
Signed-off-by: James Morris <jmorris@namei.org>