Rename PageData to XLogPageData

I was fiddling a bit with making some Page-related APIs const-proof, 
which might involve changing something like "Page p" to "const PageData 
*p", but I was surprised that a type PageData exists but it's an 
unrelated type local to generic_xlog.c.

This patch renames that type to a more specific name XLogPageData.  This 
makes room for possibly adding another PageData type with the earlier 
meaning, but that's not done here.  But I think even without that, this 
patch is a useful little cleanup that makes the code more consistent and 
clear.