md/raid10: add rcu protection to rdev access in raid10_sync_request.

mirrors[].rdev can become NULL at any point unless:
  - a counted reference is held
  - ->reconfig_mutex is held, or
  - rcu_read_lock() is held

Previously they could not become NULL during a resync/recovery/reshape either.
However when remove_and_add_spares() was added to hot_remove_disk(), that
changed.

So raid10_sync_request didn't previously need to protect rdev access,
but now it does.

Fix missed check(Shaohua)
Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Shaohua Li <shli@fb.com>