buf.h: Fix declaration of unmapped_buf

For architectures with a small-data area, the __read_mostly section must
present at the object declaration.

(emaste note: This does not appear to have an affect within FreeBSD, but
may be needed by downstream projects that handle __read_mostly /
__section(".data.read_mostly") differently.)

Pull Request:	#608

sys/sys/buf.h

@@ -522,7 +522,8 @@ extern int	bdwriteskip;
 extern int	dirtybufferflushes;
 extern int	altbufferflushes;
 extern int	nswbuf;			/* Number of swap I/O buffer headers. */
-extern caddr_t	unmapped_buf;	/* Data address for unmapped buffers. */
+extern caddr_t __read_mostly unmapped_buf; /* Data address for unmapped
+					      buffers. */
 
 static inline int
 buf_mapped(struct buf *bp)