kernel/vm: Add ASSERT_UNREACHABLE() in VMCache::Commit.

This should never be invoked; only derived-class implementations
should be.

src/system/kernel/vm/VMCache.cpp

@@ -1297,8 +1297,8 @@ VMCache::FlushAndRemoveAllPages()
 status_t
 VMCache::Commit(off_t size, int priority)
 {
-	committed_size = size;
-	return B_OK;
+	ASSERT_UNREACHABLE();
+	return B_NOT_SUPPORTED;
 }