net: skb: move enum skb_drop_reason to standalone header file

As the skb drop reasons are getting more and more, move the enum
'skb_drop_reason' and related function to the standalone header
'dropreason.h', as Jakub Kicinski suggested.

Signed-off-by: Menglong Dong <imagedong@tencent.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>