selinux: use correct type for context length

security_sid_to_context() expects a pointer to an u32 as the address
where to store the length of the computed context.

Reported by sparse:

    security/selinux/xfrm.c:359:39: warning: incorrect type in arg 4
                                    (different signedness)
    security/selinux/xfrm.c:359:39:    expected unsigned int
                                       [usertype] *scontext_len
    security/selinux/xfrm.c:359:39:    got int *

Signed-off-by: Christian Göttsche <cgzones@googlemail.com>
[PM: wrapped commit description]
Signed-off-by: Paul Moore <paul@paul-moore.com>

Author: cgzones

security/selinux/xfrm.c

@@ -347,7 +347,7 @@ int selinux_xfrm_state_alloc_acquire(struct xfrm_state *x,
 	int rc;
 	struct xfrm_sec_ctx *ctx;
 	char *ctx_str = NULL;
-	int str_len;
+	u32 str_len;
 
 	if (!polsec)
 		return 0;