bsdlabel: limit to 8 partitions

bsdlabel is intended to support up to 20 partitions, but the disklabel
struct has a d_partitions array with only BSD_NPARTS_MIN (8) entries.
Previously, an attempt to operate on a bsdlabel with more than eight
partitions resulted in a buffer overflow.

As a stopgap limit bsdlabel to 8 partitions until this is fixed
properly.

PR:		276517

sbin/bsdlabel/bsdlabel.c

@@ -50,7 +50,7 @@
 #include <sys/disk.h>
 #define DKTYPENAMES
 #define FSTYPENAMES
-#define MAXPARTITIONS	20
+#define MAXPARTITIONS	8 /* XXX should be 20, but see PR276517 */
 #include <sys/disklabel.h>
 
 #include <unistd.h>