dm: sparse: Annotate field with __rcu for checking

Annotate the map field with __rcu since this is a rcu pointer which is checked
by sparse.

Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>