[POWERPC] Remove duplicate variable definitions in mm/tlb_64.c

Somewhere along the way (e28f7faf05159f1cfd564596f5e6178edba6bd49,
"Four level pagetables for ppc64") we ended up with duplicate
definitions for pte_freelist_cur and pte_freelist_force_free.
Somehow this compiles, but it would be better to just have one
definition for each.

The two definitions we end up with can be static too!

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