devlink: fix xa_alloc_cyclic() error handling

In case of returning 1 from xa_alloc_cyclic() (wrapping) ERR_PTR(1) will
be returned, which will cause IS_ERR() to be false. Which can lead to
dereference not allocated pointer (rel).

Fix it by checking if err is lower than zero.

This wasn't found in real usecase, only noticed. Credit to Pierre.

Fixes: c137743b ("devlink: introduce object and nested devlink relationship infra")
Signed-off-by: Michal Swiatkowski <michal.swiatkowski@linux.intel.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>