p_top must be set in process.c as well for bank16k

p_top must be set in process.c as well for bank16k

I'm not exactly sure, but I think I found a bug in the bank16k
implementation.  When ptab_alloc() calls pagemap_alloc(), p_top is not
set yet.  pagemap_alloc, then proceeds to assign only one bank to the
new processes.  Assuming normal copy forking, should p_top be set to
the parent's p_top *before* calling pagemap_alloc() ?

This fix seems to make my coco3 UDATA mods start working