powerpc/mm: Make careful_allocation() return virtual addrs

Since we memset() the result in both of the uses here,
just make careful_alloc() return a virtual address.
Also, add a separate variable to store the physial
address that comes back from the lmb_alloc() functions.
This makes it less likely that someone will screw it up
forgetting to convert before returning since the vaddr
is always in a void* and the paddr is always in an
unsigned long.

I admit this is arbitrary since one of its users needs
a paddr and one a vaddr, but it does remove a good
number of casts.

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