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: 795bc112 ("IB/srpt: Make it safe to use RCU for srpt_device.rch_list")
Fixes: a1125314 ("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>