Fix misleading output from gin_desc().

XLOG_GIN_UPDATE_META_PAGE and XLOG_GIN_DELETE_LISTPAGE records were printed
with a list link field labeled as "blkno", which was confusing, especially
when the link was empty (InvalidBlockNumber).  Print the metapage block
number instead, since that's what's actually being updated.  We could
include the link values too as a separate field, but not clear it's worth
the trouble.

Back-patch to 8.4 where the dubious code was added.