kernel/vm: Do not invoke the low_resource monitor in map_backing_store.

As the comment implies, we don't want to wait here, but low_resource()
waits forever when timeout=0. It doesn't seem to do anything with the
"size" argument at all, so not calling it is just as effective, and so
replace the comment there altogether.

The case where this was most often hit, causing a full system deadlock
in anything relating to memory management, was when the passed "size"
was 0. So now we check for this case explicity at the beginning and
panic() on KDEBUG kernels for it.

src/system/kernel/vm/vm.cpp

@@ -805,6 +805,14 @@ map_backing_store(VMAddressSpace* addressSpace, VMCache* cache, off_t offset,
 		_area, areaName));
 	cache->AssertLocked();
 
+	if (size == 0) {
+#if KDEBUG
+		panic("map_backing_store(): called with size=0 for area '%s'!",
+			areaName);
+#endif
+		return B_BAD_VALUE;
+	}
+
 	uint32 allocationFlags = HEAP_DONT_WAIT_FOR_MEMORY
 		| HEAP_DONT_LOCK_KERNEL_SPACE;
 	int priority;
@@ -873,11 +881,11 @@ map_backing_store(VMAddressSpace* addressSpace, VMCache* cache, off_t offset,
 		allocationFlags, _virtualAddress);
 	if (status == B_NO_MEMORY
 			&& addressRestrictions->address_specification == B_ANY_KERNEL_ADDRESS) {
-		// Since the kernel address space is locked by the caller, we can't
-		// wait here as of course no resources can be released while the locks
-		// are held. But we can at least issue this so the next caller doesn't
-		// run into the same problem.
-		low_resource(B_KERNEL_RESOURCE_ADDRESS_SPACE, size, 0, 0);
+		// TODO: At present, there is no way to notify the low_resource monitor
+		// that kernel addresss space is fragmented, nor does it check for this
+		// automatically. Due to how many locks are held, we cannot wait here
+		// for space to be freed up, but it would be good to at least notify
+		// that we tried and failed to allocate some amount.
 	}
 	if (status != B_OK)
 		goto err2;