dm: use RCU_INIT_POINTER instead of rcu_assign_pointer in __unbind

Replace rcu_assign_pointer(p, NULL) with RCU_INIT_POINTER(p, NULL).

The rcu_assign_pointer() ensures that the initialization of a structure
is carried out before storing a pointer to that structure.  And in the
case of the NULL pointer, there is no structure to initialize.  So,
rcu_assign_pointer(p, NULL) can be safely converted to
RCU_INIT_POINTER(p, NULL).

Signed-off-by: Monam Agarwal <monamagarwal123@gmail.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>