IB/cma: Fix a race condition in iboe_addr_get_sgid()

Code that dereferences the struct net_device ip_ptr member must be
protected with an in_dev_get() / in_dev_put() pair. Hence insert
calls to these functions.

Fixes: commit 7b85627b

 ("IB/cma: IBoE (RoCE) IP-based GID addressing")
Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Reviewed-by: Moni Shoua <monis@mellanox.com>
Cc: Or Gerlitz <ogerlitz@mellanox.com>
Cc: Roland Dreier <roland@purestorage.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Doug Ledford <dledford@redhat.com>