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: NRalf Baechle <ralf@linux-mips.org>

arch/mips/kernel/scall64-n32.S

@@ -403,8 +403,8 @@ EXPORT(sysn32_call_table)
 	PTR	sys_dup3			/* 6290 */
 	PTR	sys_pipe2
 	PTR	sys_inotify_init1
-	PTR	sys_preadv
-	PTR	sys_pwritev
+	PTR	compat_sys_preadv
+	PTR	compat_sys_pwritev
 	PTR	compat_sys_rt_tgsigqueueinfo	/* 6295 */
 	PTR	sys_perf_event_open
 	PTR	sys_accept4