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kernel: avoid panic with segio and SG_FAULT segments

the problem is that segio doesnt check segment attributes
and it can't really in case of SG_FAULT which can be
inherited from pseg and toggle at any time.

so instead of returning -1 from fault into the fault$cputype
handler which then panics when fault happend kernel mode,
we jump into segio's waserror() block just like in the
demand load i/o error case (faulterror()).

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

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

@@ -921,7 +921,7 @@

 				continue;

 			j += sprint(statbuf+j, "%-6s %c%c %8p %8p %4ld\n",

 				sname[sg->type&SG_TYPE],

-				sg->type&SG_RONLY ? 'R' : ' ',

+				sg->type&SG_FAULT ? 'F' : (sg->type&SG_RONLY ? 'R' : ' '),

 				sg->profile ? 'P' : ' ',

 				sg->base, sg->top, sg->ref);

 		}

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

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

@@ -40,6 +40,8 @@

 		if((attr & SG_FAULT) != 0 || !read && (attr & SG_RONLY) != 0) {

 			qunlock(s);

 			up->psstate = sps;

+			if(up->kp && up->nerrlab)	/* for segio */

+				error(Eio);

 			return -1;

 		}