net: dropreason: reformat the comment fo skb drop reasons

To make the code clear, reformat the comment in dropreason.h to k-doc
style.

Now, the comment can pass the check of kernel-doc without warnning:

$ ./scripts/kernel-doc -v -none include/linux/dropreason.h
include/linux/dropreason.h:7: info: Scanning doc for enum skb_drop_reason

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