During pg_upgrade, conditionally skip transfer of FSMs.

During pg_upgrade, conditionally skip transfer of FSMs.

If a heap on the old cluster has 4 pages or fewer, and the old cluster
was PG v11 or earlier, don't copy or link the FSM. This will shrink
space usage for installations with large numbers of small tables.

This will allow pg_upgrade to take advantage of commit b0eaa4c51b where
we have avoided creation of the free space map for small heap relations.

Author: John Naylor
Reviewed-by: Amit Kapila
Discussion: https://postgr.es/m/CACPNZCu4cOdm3uGnNEGXivy7Gz8UWyQjynDpdkPGabQ18_zK6g%40mail.gmail.com