devlink: make all symbols GPL-only

devlink_alloc() and devlink_register() are both GPL.
A non-GPL module won't get far, so for consistency
we can make all symbols GPL without risking any real
life breakage.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>