md/raid5-ppl: Annotate with rcu_dereference_protected()

To suppress the last remaining sparse warnings about accessing
rdev, add rcu_dereference_protected calls to a couple places
in raid5-ppl. All of these places are called under raid5_run and
therefore are occurring before the array has started and is thus
safe.

There's no sensible check to do for the second argument of
rcu_dereference_protected() so a comment is added instead.

Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Song Liu <song@kernel.org>