In bootstrap mode, don't allow the creation of files if they don't already

In bootstrap mode, don't allow the creation of files if they don't already
exist.

In commit's b9d01fe288 and 3908473c80, we have added some code where we
allowed the creation of files during mdopen even if they didn't exist
during the bootstrap mode.  The later commit obviates the need for same.

This was harmless code till now but with an upcoming feature where we don't
allow to create FSM for small tables, this will needlessly create FSM
files.

Author: John Naylor
Reviewed-by: Amit Kapila
Discussion: https://www.postgresql.org/message-id/CAJVSVGWvB13PzpbLEecFuGFc5V2fsO736BsdTakPiPAcdMM5tQ@mail.gmail.com
	    https://www.postgresql.org/message-id/CAA4eK1KsET6sotf+rzOTQfb83pzVEzVhbQi1nxGFYVstVWXUGw@mail.gmail.com