Do not calculate the stack's bottom address twice.

Submitted by:	Olivц╘r Pintц╘r
Review:	https://reviews.freebsd.org/D2953
MFC after:	1 week

sys/kern/kern_exec.c

@@ -1096,7 +1096,7 @@ exec_new_vmspace(imgp, sv)
 	 * are still used to enforce the stack rlimit on the process stack.
 	 */
 	vmspace->vm_ssize = sgrowsiz >> PAGE_SHIFT;
-	vmspace->vm_maxsaddr = (char *)sv->sv_usrstack - ssiz;
+	vmspace->vm_maxsaddr = (char *)stack_addr;
 
 	return (0);
 }