This fixes a bug in the bcache code whereby false cache hits occur

the first time block 0 is read. This fix initialises the block
numbers to -1 which isn't the most correct thing for a daddr_t but
it isn't likely to cause a problem in the boot blocks and it could
do with a more thought out fix later.

The bug is probably benign on the i386 but on the alpha it can
cause initial file opens to fail. This is the cause of the "can't
open /boot/boot.conf" errors.

It appears on the alpha because of a number of combining factors.
On the alpha the LABELSECTOR is 0 so block 0 needs to be read in
from the media. The first time this happens you get a false hit
because the bc_blkno field is zero initially. Also, the timestamp
check against this cache hit succeeds because on the alpha a hacked
getsecs() function can return 0 when it starts counting so that
the zero initial timestamp + BCACHE_TIMEOUT is greater than the
current time until getsecs() has counted passed BCACHE_TIMEOUT.

The overall effect is that the first open() that occurs gets a
false cache hit and returns garbage to the bd_strategy() function
which then fails the open() call. This false hit then stays in the
cache until BCACHE_TIMEOUT getsecs() ticks have passed; all open()
calls during this time fail.

This explains why you can generally access the media by the time
you get to interp() and start issuing commands but the earlier
attempts to run the boot scripts are failing.

It's possible that this is causing the problem switching to the
mfsroot floppy as well but I haven't confirmed that.

sys/boot/common/bcache.c

@@ -23,7 +23,7 @@
  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  * SUCH DAMAGE.
  *
- *	$Id$
+ *	$Id: bcache.c,v 1.3 1998/11/04 00:29:01 msmith Exp $
  */
 
 /*
@@ -98,8 +98,10 @@ bcache_init(int nblks, size_t bsize)
     }
 
     /* Invalidate the cache */
-    for (i = 0; i < bcache_nblks; i++)
+    for (i = 0; i < bcache_nblks; i++) {
 	bcache_ctl[i].bc_count = -1;
+	bcache_ctl[i].bc_blkno = -1;
+    }
 
     return(0);
 }