security: fix typo in security_task_prctl

commit b7f76ea2ef6739ee484a165ffbac98deb855d3d3 upstream.

Signed-off-by: Jann Horn <jann@thejh.net>
Reviewed-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>