f2fs: Enable negative dentries on case-insensitive lookup

Instead of invalidating negative dentries during case-insensitive
lookups, mark them as such and let them be added to the dcache.
d_ci_revalidate is able to properly filter them out if necessary based
on the dentry casefold flag.

Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.com>
Message-Id: <20230727172843.20542-8-krisman@suse.de>
Signed-off-by: Christian Brauner <brauner@kernel.org>