Make the d_flags of mem devices D_DISK to signify that they are disk-like

random-seekable devices. This lets dd(1) know it can seek on them. It
also affects spec_vnopen() (IIRC), but only makes the path of execution smaller,
and does not change its behavior. This is when securelevel >= 2.

sys/alpha/alpha/mem.c

@@ -95,7 +95,7 @@ static struct cdevsw mem_cdevsw = {
 	/* maj */	CDEV_MAJOR,
 	/* dump */	nodump,
 	/* psize */	nopsize,
-	/* flags */	0,
+	/* flags */	D_DISK,
 	/* maxio */	0,
 	/* bmaj */	-1
 };

sys/amd64/amd64/mem.c

@@ -95,7 +95,7 @@ static struct cdevsw mem_cdevsw = {
 	/* maj */	CDEV_MAJOR,
 	/* dump */	nodump,
 	/* psize */	nopsize,
-	/* flags */	0,
+	/* flags */	D_DISK,
 	/* maxio */	0,
 	/* bmaj */	-1
 };

sys/i386/i386/mem.c

@@ -95,7 +95,7 @@ static struct cdevsw mem_cdevsw = {
 	/* maj */	CDEV_MAJOR,
 	/* dump */	nodump,
 	/* psize */	nopsize,
-	/* flags */	0,
+	/* flags */	D_DISK,
 	/* maxio */	0,
 	/* bmaj */	-1
 };