base: add Attached_dataspace::clear()

This alleviates the need (and risk) of manually parametrizing 'memset'
whenever the entire dataspace shall be cleared.

Issue #3897

repos/base/include/base/attached_ram_dataspace.h

@@ -139,6 +139,8 @@ class Genode::Attached_ram_dataspace
 		 */
 		size_t size() const { return _size; }
 
+		void clear() { if (_at) memset((void *)_at, 0, _size); }
+
 		void swap(Attached_ram_dataspace &other)
 		{
 			_swap(_size, other._size);