powerpc/mm: Cleanup careful_allocation(): consolidate memset()

Both users of careful_allocation() immediately memset() the
result.  So, just do it in one place.

Also give careful_allocation() a 'z' prefix to bring it in
line with kzmalloc() and friends.

Signed-off-by: Dave Hansen <dave@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>