md/raid10: add rcu protection to rdev access during reshape.

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

Reshape isn't always suitably careful as in the past rdev couldn't be
removed during reshape.  It can now, so add protection.

Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Shaohua Li <shli@fb.com>