Use explicit allspan list instead of
trying to find all the places where
spans might be recorded.

Free can cascade into complicated
span manipulations that move them
from list to list; the old code had the
possibility of accidentally processing
a span twice or jumping to a different
list, causing an infinite loop.

R=r
DELTA=70  (28 added, 25 deleted, 17 changed)
OCL=23704
CL=23710

src/runtime/mfixalloc.c

diff --git a/src/runtime/mfixalloc.c b/src/runtime/mfixalloc.c
index 904ca7e..dd4f3f2 100644
--- a/src/runtime/mfixalloc.c
+++ b/src/runtime/mfixalloc.c
@@ -12,10 +12,12 @@
 // Initialize f to allocate objects of the given size,
 // using the allocator to obtain chunks of memory.
 void
-FixAlloc_Init(FixAlloc *f, uintptr size, void *(*alloc)(uintptr))
+FixAlloc_Init(FixAlloc *f, uintptr size, void *(*alloc)(uintptr), void (*first)(void*, byte*), void *arg)
 {
 	f->size = size;
 	f->alloc = alloc;
+	f->first = first;
+	f->arg = arg;
 	f->list = nil;
 	f->chunk = nil;
 	f->nchunk = 0;
@@ -38,6 +40,8 @@
 		f->nchunk = FixAllocChunk;
 	}
 	v = f->chunk;
+	if(f->first)
+		f->first(f->arg, v);
 	f->chunk += f->size;
 	f->nchunk -= f->size;
 	return v;