Avoid multiple search of reserved regions during splitting

CDS uses MemTracker::record_virtual_memory_split_reserved() to split a reserved area into two parts. Later in two separate calls of MemTracker::record_virtual_memory_type()
the flags of the parts are set appropriately. Since each call to MemTracker::record_virtual_memory_type() requires NMT to search through all the reserved regions, we can save these two calls by passing the flags in the split call.