There is no need in extra call indirection and check from impossible
flow where someone tries to set namespace without prior call
to devlink_alloc().

Instead of this extra logic and additional EXPORT_SYMBOL, use specialized
devlink allocation function that receives net namespace as an argument.

Such specialized API allows clear view when devlink initialized in wrong
net namespace and/or kernel users don't try to change devlink namespace
under the hood.
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>