libata: fix n_sectors failure handling during revalidation

If revalidation fails because device has different n_sectors after
configuration the original n_sectors should be restored before failing
revalidation.  Without this fix, n_sectors difference will incorrectly
and silently pass revalidation when revalidation is retried.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Acked-by: Alan Cox <alan@redhat.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>