md/raid10: avoid fullsync when not necessary.

This is the raid10 equivalent of

commit 4f0a5e012cf41321d611e7cad63e1017d143d138
    MD RAID1: Further conditionalize 'fullsync'

If a device in a newly assembled array is not fully recovered we
currently do a fully resync by don't need to.

Signed-off-by: NeilBrown <neilb@suse.de>
1 file changed