netfs: Miscellaneous tidy ups

Do a couple of miscellaneous tidy ups:

 (1) Add a qualifier into a file banner comment.

 (2) Put the writeback folio traces back into alphabetical order.

 (3) Remove some unused folio traces.

Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org