No need to dereference struct proc to pids when comparing processes

for equality.

Reviewed by:	jhb, pho
Tested by:	pho
Sponsored by:	The FreeBSD Foundation
MFC after:	2 weeks

sys/kern/sys_process.c

@@ -749,7 +749,7 @@ kern_ptrace(struct thread *td, int req, pid_t pid, void *addr, int data)
 
 	case PT_ATTACH:
 		/* Self */
-		if (p->p_pid == td->td_proc->p_pid) {
+		if (p == td->td_proc) {
 			error = EINVAL;
 			goto fail;
 		}