incremental_container: preserve 'in_sync' flag when adding to existing array.

incremental_container: preserve 'in_sync' flag when adding to existing array.

When building container members with -IR, we need to ensure that
devices added to an active array preserve the 'in_sync' status so they
don't needlessly get rebuilt.

So allow sysfs_add_disk to do this (only works in kernels since
2.6.30) and pass the relevant flag down.

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