tcp: Fix 32 bit platform breakage

This fixes the incorrect use of a sysctl add to u64. It
was for a useconds time, but on 32 bit platforms its
not a u64. Instead use the long directive.

Reviewed by: tuexen
Sponsored by: Netflix Inc.
Differential Revision: https://reviews.freebsd.org/D31107

(cherry picked from commit 7312e4e5cfc8e48597acf17f4faa8159f0b5fa06)