bufmgr: Switch to standard order in MarkBufferDirtyHint()

bufmgr: Switch to standard order in MarkBufferDirtyHint()

When we were updating hint bits with just a share lock MarkBufferDirtyHint()
had to use a non-standard order of operations, i.e. WAL log the buffer before
marking the buffer dirty. This was required because the lock level used to set
hints did not conflict with the lock level that was used to flush pages, which
would have allowed flushing the page out before the WAL record. The
non-standard order in turn required preventing the checkpoint from starting
between writing the WAL record and flushing out the page.

Now that setting hints and writing out buffers use share-exclusive, we can
revert back to the normal order of operations.

Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi>
Discussion: https://postgr.es/m/5ubipyssiju5twkb7zgqwdr7q2vhpkpmuelxfpanetlk6ofnop@hvxb4g2amb2d