IB/ipoib: Fix RCU annotations in ipoib_neigh_hash_init()

Avoid that sparse complains about ipoib_neigh_hash_init(). This
patch does not change any functionality. See also patch "IPoIB:
Fix memory leak in the neigh table deletion flow" (commit ID
66172c09).

Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Cc: Shlomo Pongratz <shlomop@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>