RDMA/srpt: Fix RCU debug build error

Combination of CONFIG_DEBUG_OBJECTS_RCU_HEAD=y and
CONFIG_INFINIBAND_SRPT=m produces the following build error.

ERROR: "init_rcu_head" [drivers/infiniband/ulp/srpt/ib_srpt.ko] undefined!
make[1]: *** [scripts/Makefile.modpost:92: __modpost] Error 1
make: *** [Makefile:1216: modules] Error 2

The reason to it that init_rcu_head() is not exported and not supposed
to be used in modules. It is needed for dynamic initialization of
statically allocated rcu_head structures.

Fixes: 795bc112cd5a ("IB/srpt: Make it safe to use RCU for srpt_device.rch_list")
Fixes: a11253142e6d ("IB/srpt: Rework multi-channel support")
Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>

drivers/infiniband/ulp/srpt/ib_srpt.c

@@ -1950,7 +1950,6 @@ static struct srpt_nexus *srpt_get_nexus(struct srpt_port *sport,
 			nexus = ERR_PTR(-ENOMEM);
 			break;
 		}
-		init_rcu_head(&tmp_nexus->rcu);
 		INIT_LIST_HEAD(&tmp_nexus->ch_list);
 		memcpy(tmp_nexus->i_port_id, i_port_id, 16);
 		memcpy(tmp_nexus->t_port_id, t_port_id, 16);
@@ -2110,7 +2109,6 @@ static int srpt_cm_req_recv(struct ib_cm_id *cm_id,
 		goto reject;
 	}
 
-	init_rcu_head(&ch->rcu);
 	kref_init(&ch->kref);
 	ch->pkey = be16_to_cpu(pkey);
 	ch->nexus = nexus;