Currently, zap_remove()/zap_remove_by_dnode() will lookup the specified index (or name) to locate the entry internally, then remove the found entry. In fact, for most of cases, the caller has already performed lookup before calling zap_remove()/zap_remove_by_dnode(). But related lookup result cannot be reused by the zap_remove()/zap_remove_by_dnode(). If we can extend ZAP remove API to recognize the former lookup result, and remove the specified entry directly, then the unlink/delete performance can be improved.