kernel/vm: Add "unmergeable" flag to VMCache.

ramfs needs to create caches that are both temporary and unmergeable,
so add another flag to make this state possible.

Otherwise, mmap'ed files from ramfs might wind up in VMCache
trying to merge the caches when the last one is closed, which
we don't want.

headers/private/kernel/vm/VMCache.h

@@ -198,6 +198,7 @@ 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

@@ -255,6 +255,7 @@ DataContainer::_SwitchToCacheMode()
 		return error;
 
 	fCache->temporary = 1;
+	fCache->unmergeable = 1;
 	fCache->virtual_end = fSize;
 
 	error = fCache->Commit(fSize, VM_PRIORITY_USER);

src/system/kernel/vm/VMCache.cpp

@@ -627,8 +627,8 @@ VMCacheRef::VMCacheRef(VMCache* cache)
 bool
 VMCache::_IsMergeable() const
 {
-	return areas == NULL && temporary && !consumers.IsEmpty()
+	return areas == NULL && temporary && !unmergeable
-		&& consumers.Head() == consumers.Tail();
+		&& !consumers.IsEmpty() && consumers.Head() == consumers.Tail();
 }
 
 
@@ -657,6 +657,7 @@ VMCache::Init(uint32 cacheType, uint32 allocationFlags)
 	virtual_end = 0;
 	committed_size = 0;
 	temporary = 0;
+	unmergeable = 0;
 	page_count = 0;
 	fWiredPagesCount = 0;
 	type = cacheType;