devlink: un-static devlink_nl_pre/post_doit()

To be prepared for the follow-up generated split ops addition,
make the functions devlink_nl_pre_doit() and devlink_nl_post_doit()
usable outside of netlink.c. Introduce temporary prototypes which are
going to be removed once the generated header will be included.

Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Link: https://lore.kernel.org/r/20230803111340.1074067-9-jiri@resnulli.us


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