make type of st_dev explicitly dev_t in x86_64 stat.h

otherwise it's unclear that it's correct. aside from that, it makes
for a gratuitous difference between the x86_64 header and the upcoming
x32 header.

arch/x86_64/bits/stat.h

@@ -2,7 +2,7 @@
  * by the corresponding correctly-sized userspace types. */
 
 struct stat {
-	unsigned long st_dev;
+	dev_t st_dev;
 	ino_t st_ino;
 	nlink_t st_nlink;