New RESHAPE_NO_BACKUP flag to track when backup action is needed.
Some arrays (raid10) never need a backup file, so during assembly
we can avoid the whole Grow_continue check in that case.
Achieve this using a flag set by the metadata handler.
Also get "mdadm -I" to fail if a backup process would be
needed. It currently does fail as the kernel rejects things,
but it is nicer to have this explicit.