xfsdump: getdents.c: clean up ifdef-hell around sys_getdents

Remove the ifdef-hell imported from glibc around getdents64. Everything
since 2.4.1 supports things properly and this seems like a sensible
clean up, so lets just assume it exists and handle it by falling back
if not.

Additionally, if the old getdents syscall doesn't exist, just stub out
the actual syscall and return ENOSYS.

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