tcp: Simplify the initialization of loader tunable 'net.inet.tcp.tcbhashsize'

No functional change intended.

Reviewed by: cc, rscheff, transport
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D41998

(cherry picked from commit 38ecc80b2a4e5e11ece83ca4df63632f0b6fa394)