cons: Clamp the amount written to the console

Large writes to the console take a long time.  Serial writes of about a
page take O(100ms).  Our kernel is currently non-preemptive, which is
based on a model of syscalls not taking too long.

Limiting the write sizes to 80 (the width of an ancient console, which
Ron will love) cuts the syscall time down to a few ms, which is within
the quantum of a process.

Signed-off-by: Barret Rhoden <brho@cs.berkeley.edu>

kern/drivers/dev/cons.c

diff --git a/kern/drivers/dev/cons.c b/kern/drivers/dev/cons.c
index b2639da..90d9890 100644
--- a/kern/drivers/dev/cons.c
+++ b/kern/drivers/dev/cons.c
@@ -1226,8 +1226,10 @@
 			 * keyboard would try to print it (which it can't do yet).  The hack
 			 * is even dirtier in that we only detect it if it is the first
 			 * char, and we ignore everything else.  \033 is 0x1b. */
-			if (((char*)va)[0] != '\033')
+			if (((char*)va)[0] != '\033') {
+				n = MIN(n, 80);
 				cputbuf(va, n);
+			}
 			poperror();
 			px_unlock();
 			return n;