securityfs: fix securityfs_create_dir comment

If there is an error creating a directory with securityfs_create_dir,
the error is propagated via ERR_PTR but the function comment claims that
NULL is returned.

This is a similar commit to 88e6c94cda322ff2b32f72bb8d96f9675cdad8aa
("fix long-broken securityfs_create_file comment") that did not fix
securityfs_create_dir comment at the same time.

Signed-off-by: Laurent Georget <laurent.georget@supelec.fr>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>