Grow.c: Block any level migration with chunk size change

Grow.c: Block any level migration with chunk size change

Mixing level and chunk changes in one grow operation is not supported.
Mdadm performs level migration correctly and ignores new chunk, but
after migration it tries to write this chunk to sysfs properties.
This is dangerous and can cause unexpected behaviours.

Block it before level migration starts.

Signed-off-by: Mariusz Tkaczyk <mariusz.tkaczyk@intel.com>
Signed-off-by: Jes Sorensen <jsorensen@fb.com>