[PATCH] powerpc: Only calculate htab_size in one place for kexec

For kexec we need to know the size of the MMU hash table.

Currently we calculate the size once in the htab code, and then twice more in
the kexec code, once using htab_hash_mask and once using ppc64_pft_size.
On some machines the ppc64_pft_size calculation is broken because
ppc64_pft_size is not set.

So we need to fix the second calculation, but better still we should just
calculate the size once and use it everywhere else.

Tested on Power5 LPAR, Power4 non-LPAR and Power3.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>