vfs: retire user_path_at_empty and drop empty arg from getname_flags

No users after do_readlinkat started doing the job on its own.

Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
Link: https://lore.kernel.org/r/20240604155257.109500-3-mjguzik@gmail.com


Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>