libfs: Create the helper function generic_ci_validate_strict_name()

Create a helper function for filesystems do the checks required for
casefold directories and strict encoding.

Suggested-by: Gabriel Krisman Bertazi <krisman@suse.de>
Reviewed-by: Gabriel Krisman Bertazi <gabriel@krisman.be>
Signed-off-by: André Almeida <andrealmeid@igalia.com>
Link: https://lore.kernel.org/r/20241021-tonyk-tmpfs-v8-1-f443d5814194@igalia.com


Signed-off-by: Christian Brauner <brauner@kernel.org>