Fix stack size in plumb.

Increasing the stack size to 16KB prevents the plumber
to crash in some situations on the AMD64 kernel.

112.4 (*t)->magic == FREE_MAGIC: assertion failed
plumber 112: suicide: sys: trap: fault read addr=0x0 pc=0x20c5ec

Erik Quanstrom did the original fix.

Reference: /n/sources/patch/applied/plumb-stacksize
Date: Wed Feb 20 07:02:56 CET 2013
Signed-off-by: djc@9grid.fr

--- /sys/src/cmd/plumb/fsys.c	Wed Feb 20 07:02:38 2013
+++ /sys/src/cmd/plumb/fsys.c	Wed Feb 20 07:02:34 2013
@@ -10,7 +10,7 @@
 
 enum
 {
-	Stack = 8*1024
+	Stack = 16*1024
 };
 
 typedef struct Dirtab Dirtab;