md: make reconfig_mutex optional for writes to md sysfs files.

Rather than using mddev_lock() to take the reconfig_mutex
when writing to any md sysfs file, we only take mddev_lock()
in the particular _store() functions that require it.
Admittedly this is most, but it isn't all.

This also allows us to remove special-case handling for new_dev_store
(in md_attr_store).

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