tcp: fix format of sysctl variable

The format for CTLTYPE_UINT is "IU" instead of "UI" as specified
in sysctl.9.

Reviewed by: cc, zlei
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D46408

(cherry picked from commit 498286d4e807d6b9e4caad22b96ebca7f16e9b18)