devlink: Enable the use of private flags in post_doit operations

Currently, private flags (e.g., 'DEVLINK_NL_FLAG_NEED_PORT') are only
used in pre_doit operations, but a subsequent patch will need to
conditionally lock and unlock the device lock in pre and post doit
operations, respectively.

As a preparation, enable the use of private flags in post_doit
operations in a similar fashion to how it is done for pre_doit
operations.

No functional changes intended.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>