kernel: demand_paging: clear BSS after paging is initialized

If BSS section is not present in memory at boot, it would not
have been cleared as the data pages are not in physical memory.
Manipulating those pages would result in page faults.
In this scenario, zeroing BSS can only be done once the paging
mechanism has been initialized. So do it there.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>