linux-boot: Make more disk partition block devices.

Reported by nebuli on #guix.

* gnu/build/linux-boot.scm (make-disk-device-nodes): Make 16 block
  devices for disk partitions.

gnu/build/linux-boot.scm

@@ -93,7 +93,7 @@ (define* (make-disk-device-nodes base major #:optional (minor 0))
 with the given MAJOR number, starting with MINOR."
   (mknod base 'block-special #o644 (device-number major minor))
   (let loop ((i 1))
-    (when (< i 6)
+    (when (< i 16)
       (mknod (string-append base (number->string i))
              'block-special #o644 (device-number major (+ minor i)))
       (loop (+ i 1)))))