net: move dropreason.h to dropreason-core.h

This will, after the next patch, hold only the core
drop reasons and minimal infrastructure. Fix a small
kernel-doc issue while at it, to avoid the move
triggering a checker.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>