mdstat: Rework mdstat external arrays handling

mdstat: Rework mdstat external arrays handling

To avoid repeating mdstat_read() in IncrementalRemove(), new function
mdstat_find_by_member_name() has been proposed. With that,
IncrementalRemove() handles own copy of mdstat content and there is no
need to repeat reading for external stop.

Additionally, It proposed few helper to avoid repeating
mdstat_ent->metadata_version checks across code.

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