md/linear: remove rcu protections in favour of suspend/resume

The use of 'rcu' to protect accesses to ->private_data so that
the ->private_data could be updated predates the introduction
of mddev_suspend/mddev_resume.
These are a cleaner mechanism for providing stability while
swapping in a new ->private data - it is used by level_store()
to support changing of raid levels.

So get rid of the RCU stuff and just use mddev_suspend, mddev_resume.

As these function call ->quiesce(), we add an empty function for
linear just like for raid0.

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