remove ancient sys_getdents code paths

Everything since 2.4.1 has supported getdents64... so let's remove all
the legacy code paths to handle it and just rely on getdents64 existing
everywhere. Also re-indent the function to not look entirely awful.

Signed-off-by: Kyle McMartin <kmcmarti@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dave Chinner <david@fromorbit.com>