Merge branch 'reorganize-the-code-of-the-enum-skb_drop_reason'

Menglong Dong says:

====================
reorganize the code of the enum skb_drop_reason

The code of skb_drop_reason is a little wild, let's reorganize them.
Three things and three patches:

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

2) use auto-generation to generate the source file that convert enum
   skb_drop_reason to string.

3) make the comment of skb drop reasons kernel-doc style.

Changes since v3:
3/3: remove some useless comment (Jakub Kicinski)

Changes since v2:
2/3: - add new line in the end of .gitignore
     - fix awk warning by make '\;' to ';', as ';' is not need to be
       escaped
     - export 'drop_reasons' in skbuff.c

Changes since v1:
1/3: move dropreason.h from include/linux/ to include/net/ (Jakub Kicinski)
2/3: generate source file instead of header file for drop reasons string
     array (Jakub Kicinski)
3/3: use inline comment (Jakub Kicinski)
====================

Link: https://lore.kernel.org/r/20220606022436.331005-1-imagedong@tencent.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>