UBI: vtbl: Use ubi_eba_atomic_leb_change()

This is more a cosmetic change than a fix.
By using ubi_eba_atomic_leb_change()
we can guarantee that the first VTBL record is always
correct and we don't really need the second one anymore.
But we have to keep the second one to not break anything.

Artem: add a comment

Signed-off-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>