Merge branch 'net-create-a-net-core-internal-header'

Jakub Kicinski says:

====================
net: create a net/core/ internal header

We are adding stuff to netdevice.h which really should be
local to net/core/. Create a net/core/dev.h header and use it.
Minor cleanups precede.
====================

Link: https://lore.kernel.org/r/20220406213754.731066-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>