Fix description of WAL record XLOG_BTREE_META_CLEANUP

Fix description of WAL record XLOG_BTREE_META_CLEANUP

This record uses one metadata buffer and registers some data associated
to the buffer, but when parsing the record for its description a direct
access to the record data was done, but there is none.  This leads
usually to an incorrect description, but can also cause crashes like in
pg_waldump.  Instead, fix things so as the parsing uses the data
associated to the metadata block.

This is an oversight from 3d92796, so backpatch down to 11.

Author: Michael Paquier
Description: https://postgr.es/m/20190617013059.GA3153@paquier.xyz
Backpatch-through: 11