Fix check for child column generation status matching parent.

Fix check for child column generation status matching parent.

In commit 8bf6ec3ba, I mistakenly supposed that MergeAttributes'
loop over saved_schema was reprocessing column definitions that
had already been checked earlier: there is a variant syntax for
creating a child partition in which that's not true.  So we need
to duplicate the full check appearing further up.

(Actually, I believe that the "if (restdef->identity)" part is
not reachable, because we reject identity on partitions earlier.
But it seems wise to keep the check, in case that's ever relaxed,
and to keep this code in sync with the other instance.)

Per report from Alexander Lakhin.

Discussion: https://postgr.es/m/4a8200ca-8378-653e-38ed-b2e1f1611aa6@gmail.com