MIPS: N32: Fix preadv(2) and pwritev(2) entry points.

By using the native syscall entry point the kernel was also expecting
64-bit iovec structures.

This is broken since ddd9e91b [preadv/
pwritev: MIPS: Add preadv(2) and pwritev(2) syscalls.] which originally
added these two syscalls.  I walked through piles of code, including
libc and couldn't find anything that would have worked around the issue
so this change the API to what it should always have been.

Noticed and patch suggested by Al Viro.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>