Rename PageData to GenericXLogPageData

Rename PageData to GenericXLogPageData

In the PostgreSQL C type naming schema, the type PageData should be
what the pointer of type Page points to.  But in this case it's
actually an unrelated type local to generic_xlog.c.  Rename that to a
more specific name.  This makes room to possible add a PageData type
with the mentioned meaning, but this is not done here.

Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://www.postgresql.org/message-id/flat/001d457e-c118-4219-8132-e1846c2ae3c9%40eisentraut.org