devlink: Fix some error codes

These paths don't set the error codes.  It's especially important in
devlink_nl_region_notify_build() where it leads to a NULL dereference in
the caller.

Fixes: 544e7c33 ("net: devlink: Add support for port regions")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/20201026080059.GA1628785@mwanda


Signed-off-by: Jakub Kicinski <kuba@kernel.org>