util: use PROC_SELF in vaddr_to_pfn() for open_proc()

Just a little optimization (one syscall less).

travis-ci: success for Rectify 32-bit compatible C/R on x86
Signed-off-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

criu/util.c

@@ -768,7 +768,7 @@ int vaddr_to_pfn(unsigned long vaddr, u64 *pfn)
 	int fd, ret = -1;
 	off_t off;
 
-	fd = open_proc(getpid(), "pagemap");
+	fd = open_proc(PROC_SELF, "pagemap");
 	if (fd < 0)
 		return -1;