powerpc/mm: Silence unused-but-set-variable warnings

pte_unmap() compiles away on some powerpc platforms, so silence the
warnings below by making it a static inline function.

  mm/memory.c: In function 'copy_pte_range':
  mm/memory.c:820:24: warning: variable 'orig_dst_pte' set but not used
  mm/memory.c:820:9: warning: variable 'orig_src_pte' set but not used
  mm/madvise.c: In function 'madvise_free_pte_range':
  mm/madvise.c:318:9: warning: variable 'orig_pte' set but not used
  mm/swap_state.c: In function 'swap_ra_info':
  mm/swap_state.c:634:15: warning: variable 'orig_pte' set but not used

Suggested-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Qian Cai <cai@lca.pw>
Reviewed-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>