imsm: fix reserved sectors for spares

imsm: fix reserved sectors for spares

Different OROMs reserve different amounts of space for the migration area.
When activating a spare minimize the reserved space otherwise a valid spare
can be prevented from joining an array with a migration area smaller than
IMSM_RESERVED_SECTORS.

This may result in an array that cannot be reshaped, but that is less
surprising than not being able to rebuild a degraded array.
imsm_reserved_sectors() already reports the minimal value which adds to
the confusion when trying rebuild an array because mdadm -E indicates
that the device has enough space.

Cc: Anna Czarnowska <anna.czarnowska@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: NeilBrown <neilb@suse.de>