BRIN: fix bug in xlog backup block counting

The code that generates the BRIN_XLOG_UPDATE removes the buffer
reference when the page that's target for the updated tuple is freshly
initialized.  This is a pretty usual optimization, but was breaking the
case where the revmap buffer, which is referenced in the same WAL
record, is getting a backup block: the replay code was using backup
block index 1, which is not valid when the update target buffer gets
pruned; the revmap buffer gets assigned 0 instead.  Make sure to use the
correct backup block index for revmap when replaying.

Bug reported by Fujii Masao.