kernel/vm: Rework ordering of _RemoveConsumer and drop "unmergeable" flag.

This reverts a8877df135.

Previously, the "unmergeable" flag was necessary for the RAMFS,
because if the last vnode reference was released while there
was still a consumer (as the old ordering of _RemoveConsumer
had), then the release of the cache reference when the vnode
was removed would result in the cache trying to merge with
its now-only consumer and sole referrer.

Now, instead, we remove the consumer before releasing the store
reference, so that there's no chance the cache will be merged
inside this method.

mmap_cut_tests still pass, web browsers using ramfs shared_memory
still seem to work.

headers/private/kernel/vm/VMCache.h

@@ -200,7 +200,6 @@ public:
 				// TODO: Remove!
 			uint32				page_count;
 			uint32				temporary : 1;
-			uint32				unmergeable : 1;
 			uint32				type : 6;
 
 #if DEBUG_CACHE_LIST

src/add-ons/kernel/file_systems/ramfs/DataContainer.cpp

@@ -303,7 +303,6 @@ DataContainer::_SwitchToCacheMode()
 
 	fCache = cache;
 	fCache->temporary = 1;
-	fCache->unmergeable = 1;
 	fCache->virtual_end = fSize;
 
 	error = fCache->Commit(fSize, VM_PRIORITY_USER);

src/system/kernel/vm/VMCache.cpp

@@ -611,7 +611,7 @@ VMCacheRef::VMCacheRef(VMCache* cache)
 bool
 VMCache::_IsMergeable() const
 {
-	return areas.IsEmpty() && temporary && !unmergeable
+	return areas.IsEmpty() && temporary
 		&& !consumers.IsEmpty() && consumers.Head() == consumers.Tail();
 }
 
@@ -642,7 +642,6 @@ VMCache::Init(uint32 cacheType, uint32 allocationFlags)
 	virtual_end = 0;
 	committed_size = 0;
 	temporary = 0;
-	unmergeable = 0;
 	page_count = 0;
 	fWiredPagesCount = 0;
 	type = cacheType;
@@ -1561,21 +1560,22 @@ void
 VMCache::_RemoveConsumer(VMCache* consumer)
 {
 	TRACE(("remove consumer vm cache %p from cache %p\n", consumer, this));
-	consumer->AssertLocked();
-
 	T(RemoveConsumer(this, consumer));
 
-	// Remove the store ref before locking the cache. Otherwise we'd call into
+	consumer->AssertLocked();
-	// the VFS while holding the cache lock, which would reverse the usual
-	// locking order.
-	ReleaseStoreRef();
 
-	// remove the consumer from the cache, but keep its reference until later
+	// Remove the consumer from the cache, but keep its reference until the end.
 	Lock();
 	consumers.Remove(consumer);
 	consumer->source = NULL;
+	Unlock();
 
-	ReleaseRefAndUnlock();
+	// Release the store ref without holding the cache lock, as calling into
+	// the VFS while holding the cache lock would reverse the usual locking order.
+	ReleaseStoreRef();
+
+	// Now release the consumer's reference.
+	ReleaseRef();
 }