socket: wrap ktrsplice call with KTRACE ifdef

This fixes a build error when the kernel is built without KTRACE
support.

Reviewed by: emaste, markj
Fixes: a1da7dc1cdad ("socket: Implement SO_SPLICE")
Pull Request: https://github.com/freebsd/freebsd-src/pull/1426

(cherry picked from commit 75cd1e534c75fbdd3294debe6edea27e780bc5f1)