powerpc/mm: Fix "sz" set but not used warning

Fix compiler warning:
  arch/powerpc/mm/hugetlbpage-hash64.c: In function '__hash_page_huge':
  arch/powerpc/mm/hugetlbpage-hash64.c:29:28: warning: variable 'sz' set
  but not used [-Wunused-but-set-variable]

mpe: The last usage of sz was removed in 0895ecda

 ("powerpc/mm:
Bring hugepage PTE accessor functions back into sync with normal
accessors").

Signed-off-by: Qian Cai <cai@lca.pw>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>