Correct the used of snprintf in owinterface_add

[vax-r]

• origin size put in snprintf in function owinterface_add was wrong
• correct the size from ETH_ALEN to ETH_ALEN - 1

The origin size value in snprintf of owinterface_add was ETH_ALEN, which will only allow ETH_ALEN - 1 characters to write in the buffer when string size is too long.

However, since the buffer starts from inf_name + 1, which only allow ETH_ALEN - 1 characters to be written into, plus the last character must be '\0', so we should only allow ETH_ALEN - 2 character to be written into the buffer.

That's why I create the PR and fix the size value to ETH_ALEN - 1

[vax-r]

Read the article How to Write a Git Commit Message and rewrite your git commit messages.

I've made some modification to my commit message, please take a look when you have available time. Thanks

[jserv]

Thank @vax-r for contributing!