powerpc: Use generic compat_sys_old_readdir

Use the generic compat_sys_old_readdir instead of the powerpc one which
is almost the same except for the almost complete lack of error
handling.

Note that we can't just use SYSCALL() in systbl.h because the native
syscall is named old_readdir, not sys_old_readdir.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Paul Mackerras <paulus@samba.org>