mdadm: drop get_required_spare_criteria()

mdadm: drop get_required_spare_criteria()

Only IMSM implements get_spare_criteria, so load_super() in
get_required_spare_criteria() is dead code. It is moved inside
metadata handler, because only IMSM implements it.

Give possibility to provide devnode to be opened. With that we can hide
load_container() used only to fill spare criteria inside handler
and simplify implementation in generic code.

Add helper function for testing spare criteria in Incremental and
error messages.

File descriptor in get_spare_criteria_imsm() is always opened on purpose.
New functionality added in next patches will require it. For the same
reason, function is moved to other place.

No functional changes.

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