xfs_io: configure tests for preadv/pwritev & sync_file_range

xfs_io: configure tests for preadv/pwritev & sync_file_range

On older systems we may not have preadv/pwritev and/or
sync_file_range.

Add the configure magic, and stub out the code
where needed.

(sync_file_range just needed a better test; preadv/pwritev
took a little more rearranging)

And fix a couple typos ("numberic") while we're at it.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Mark Tinguely <tinguely@sgi.com>