allocateForCompact() is called when nursery of a compact region is
full, to add new blocks to the compact. New blocks added to an existing
region needs a StgCompactNFDataBlock header, not a StgCompactNFData.

This fixes allocateForCompact() so that it now correctly allocates space
for StgCompactNFDataBlock instead of StgCompactNFData as before.

Fixes #17044.

A regression test T17044 added.

(cherry picked from commit 981d3b9e)