MDEV-29996 Duplicated call to buf_page_t::set_ibuf_exist() on recovery

mlog_init_t::mark_ibuf_exist(): After applying the changes,
invoke clear().

It turns out that multiple calls to recv_sys.apply(true) are
possible during recovery. Therefore, we might redundantly
invoke mlog_init_t::mark_ibuf_exist() multiple times.

Starting with commit aaef2e1
the call buf_page_t::set_ibuf_exist() is not idempotent,
because the flag is actually represented by 2 values of a
3-bit state field.

storage/innobase/log/log0recv.cc

@@ -1148,6 +1148,7 @@ class mlog_init_t
 		}
 
 		mtr.commit();
+		clear();
 	}
 
 	/** Clear the data structure */