buf_page_get_zip(): Deduplicate some code

storage/innobase/buf/buf0buf.cc

@@ -3782,18 +3782,10 @@ err_exit:
 	ut_ad(!buf_pool_watch_is_sentinel(buf_pool, bpage));
 
 	switch (buf_page_get_state(bpage)) {
-	case BUF_BLOCK_POOL_WATCH:
-	case BUF_BLOCK_NOT_USED:
-	case BUF_BLOCK_READY_FOR_USE:
-	case BUF_BLOCK_MEMORY:
-	case BUF_BLOCK_REMOVE_HASH:
-		ut_error;
-
 	case BUF_BLOCK_ZIP_PAGE:
 	case BUF_BLOCK_ZIP_DIRTY:
 		buf_block_fix(bpage);
 		block_mutex = &buf_pool->zip_mutex;
-		mutex_enter(block_mutex);
 		goto got_block;
 	case BUF_BLOCK_FILE_PAGE:
 		/* Discard the uncompressed page frame if possible. */
@@ -3808,16 +3800,16 @@ err_exit:
 				      __FILE__, __LINE__);
 
 		block_mutex = &((buf_block_t*) bpage)->mutex;
-
-		mutex_enter(block_mutex);
-
 		goto got_block;
+	default:
+		break;
 	}
 
 	ut_error;
 	goto err_exit;
 
 got_block:
+	mutex_enter(block_mutex);
 	must_read = buf_page_get_io_fix(bpage) == BUF_IO_READ;
 
 	rw_lock_s_unlock(hash_lock);