IB/core: Fix static analysis warning in ib_policy_change_task

ib_get_cached_subnet_prefix can technically fail, but the only way it
could is not possible based on the loop conditions. Check the return
value before using the variable sp to resolve a static analysis warning.

-v1:
- Fix check to !ret. Paul Moore

Fixes: 8f408ab6 ("selinux lsm IB/core: Implement LSM notification
system")
Signed-off-by: Daniel Jurgens <danielj@mellanox.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>
Signed-off-by: James Morris <james.l.morris@oracle.com>