powerpc: wire up preadv2 and pwritev2 syscalls

Wire up preadv2/pwritev2 in the same way as preadv/pwritev. Fixes two
build warnings on ppc64.

mpe: Lightly tested with fio (slightly hacked to add the syscall
wrappers):

  fio-4217  [009] ....  1304.635300: sys_preadv2(fd: 3, vec:
  10025821de0, vlen: 1, pos_l: 6253000, pos_h: 0, flags: 1)
  fio-4217  [009] ....  1304.635474: sys_preadv2 -> 0x1000

Signed-off-by: Rui Salvaterra <rsalvaterra@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>