pgsql: 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

Branch
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REL_11_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/6dfc9464472ec283cf6b13ee67b42afc6346c533

Modified Files
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src/backend/access/rmgrdesc/nbtdesc.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)