if_stf: style(9) pass

As stated in style(9): "Values in return statements should be enclosed
in parentheses."

MFC after: 3 weeks
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D32848

(cherry picked from commit 3576121c8b1fcaa31b3a45ea52ff83f7c5855266)