devlink: Allocate devlink directly in requested net namespace

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>