Change way that reshaping arrays with external-metadata are assembled.

Change way that reshaping arrays with external-metadata are assembled.

Now that the external metadata handler must provide an md-compatible
old/new geometry, sys_set_array can do all of the array set-up for
an array that is undergoing reshape.
That leave less for reshape_array to do.

Also clean up how reshape_array tells if the reshape has started or
not.
Don't use ->reshape_active as that doesn't tell us anything consistent
at this stage, only use the 'restart' flag passed in.

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