Follow-up r295924: Only sync hash-based db files open for writing when closing.

This fixes a major performance regression when reading db files such as
the pw database during a 'pkg install'.

MFC after:	1 week
Tested by:	bapt
Reviewed by:	bapt
Sponsored by:	EMC / Isilon Storage Division
Differential Revision:	https://reviews.freebsd.org/D5868

lib/libc/db/hash/hash.c

@@ -423,7 +423,8 @@ hdestroy(HTAB *hashp)
 		free(hashp->tmp_buf);
 
 	if (hashp->fp != -1) {
-		(void)_fsync(hashp->fp);
+		if (hashp->save_file)
+			(void)_fsync(hashp->fp);
 		(void)_close(hashp->fp);
 	}