reshape: support raid5 grow on certain older kernels.

reshape: support raid5 grow on certain older kernels.

Kernels between
  c6563a8c38fde3c1c7fc925a v3.5-rc1~110^2~53
and
  b5254dd5fdd9abcacadb5101 v3.5-rc1~110^2~51

allow new_offset to be set, but don't then allow a RAID5
to be reshaped to change that offset.
Due to selective backports, this includes the SLES11-SP3 kernel.

It is quite easy to handle this case in mdadm, so we do.
Specifically: if the reshape with data-offset fails with EINVAL,
abort the data-offset change and try the "old" way.

Signed-off-by: NeilBrown <neilb@suse.de>