lib: Add stale 'raid6' directory to .gitignore file

lib: Add stale 'raid6' directory to .gitignore file

I keep having to do this, because people think they can just move
directories around and move the gitignore files around with them.

You really can't do that - the old generated files stay around for
others, and still need to be ignored in the old location.

So when moving gitignore entries around because you moved the files (or
when moving a whole gitignore file around because the directory it was
in moved), the old gitignore situation needs to be dealt with.

Yes, those files may have moved in *your* tree when you moved the
directory.  And yes, new repositories will never even have seen them.
But all those other developers that see the result of your move still
likely have a working tree with the old state, and the files that were
hidden from git by an old gitignore file do not suddenly become
relevant.

Fixes: 3626738bc714 ("raid6: move to lib/raid/")
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>