md: allow upper limit for resync/reshape to be set when array is read-only

Normally we only allow the upper limit for a reshape to be decreased
when the array not performing a sync/recovery/reshape, otherwise there
could be races.  But if an array is part-way through a reshape when it
is assembled the reshape is started immediately leaving no window
to set an upper bound.

If the array is started read-only, the reshape will be suspended until
the array becomes writable, so that provides a window during which it
is perfectly safe to reduce the upper limit of a reshape.

So: allow the upper limit (sync_max) to be reduced even if the reshape
thread is running, as long as the array is still read-only.

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