mdadm: pass struct context for external reshapes

mdadm: pass struct context for external reshapes

This patch alters mutiple functions calls so the context is passed to
external reshape functions.

There are two main reasons behind it:
- reduces number of arguments passed and unifies them,
- imsm code will make use of context in incoming patches.

Signed-off-by: Mateusz Kusiak <mateusz.kusiak@intel.com>
Signed-off-by: Mariusz Tkaczyk <mariusz.tkaczyk@linux.intel.com>