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devmnt: abandon fid on botched Tclunk or Tremove

if theres an error transmitting a Tclunk or Tremove request,
we cannot assume the fid to be clunked. in case this was
a transient error, reusing the fid on further requests
will fail.

as a work arround, we zero the channels fid and allocate
a new fid before the chan is reused.

this is not correct as we essentially leak the fid
on the fileserver, but we will still be able to use
the mount.

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

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

@@ -225,20 +225,19 @@

 	lock(&chanalloc);

 	c = chanalloc.free;

-	if(c != 0){

+	if(c != nil){

 		chanalloc.free = c->next;

-		c->next = 0;

-	}

-	unlock(&chanalloc);

-

-	if(c == nil){

+		c->next = nil;

+	} else {

+		unlock(&chanalloc);

 		c = smalloc(sizeof(Chan));

 		lock(&chanalloc);

-		c->fid = ++chanalloc.fid;

 		c->link = chanalloc.list;

 		chanalloc.list = c;

-		unlock(&chanalloc);

 	}

+	if(c->fid == 0)

+		c->fid = ++chanalloc.fid;

+	unlock(&chanalloc);

 	/* if you get an error before associating with a dev,

 	   close calls rootclose, a nop */

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

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

@@ -777,6 +777,13 @@

 		if(m->rip == up)

 			mntgate(m);

 		if(strcmp(up->errstr, Eintr) != 0){

+			switch(r->request.type){

+			case Tremove:

+			case Tclunk:

+				/* botch, abandon fid */ 

+				if(strcmp(up->errstr, Ehungup) != 0)

+					r->c->fid = 0;

+			}

 			mntflushfree(m, r);

 			nexterror();

 		}