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kernel: pass segio error string by pointer

there's no need to waste space for a error buffer in the Segio
structure, as the segmentio kproc will be waiting for the next
command after an error and will not overwite it until we issue
another command.

--- a/sys/src/9/port/portdat.h

+++ b/sys/src/9/port/portdat.h

@@ -406,7 +406,7 @@

 	char	*addr;

 	int	dlen;

 	int	cmd;

-	char	err[64];

+	char	*err;

 };

 enum

--- a/sys/src/9/port/segment.c

+++ b/sys/src/9/port/segment.c

@@ -797,11 +797,11 @@

 static void

 docmd(Segio *sio, int cmd)

 {

-	sio->err[0] = 0;

+	sio->err = nil;

 	sio->cmd = cmd;

 	wakeup(&sio->cmdwait);

 	sleep(&sio->replywait, cmddone, sio);

-	if(sio->err[0])

+	if(sio->err != nil)

 		error(sio->err);

 }

@@ -838,10 +838,9 @@

 		;

 	for(done = 0; !done;){

 		sleep(&sio->cmdwait, cmdready, sio);

-		if(waserror()){

-			strncpy(sio->err, up->errstr, sizeof(sio->err)-1);

-			sio->err[sizeof(sio->err)-1] = 0;

-		} else {

+		if(waserror())

+			sio->err = up->errstr;

+		else {

 			if(sio->s != nil && up->seg[sno] != sio->s){

 				putseg(up->seg[sno]);

 				incref(sio->s);