MAC: mac_biba, mac_lomac: Fix setting loader tunables

A string loader tunable requires setting the len parameter to a nonzero
value, typically the size of the string, to have the flag CTLFLAG_TUN
work correctly [1] [2].

Without this fix security.mac.{biba,lomac}.trusted_interfaces would
have no effect at all.

[1] 3da1cf1e88f8 Extend the meaning of the CTLFLAG_TUN flag to automatically ...
[2] 6a3287f889b0 Fix regression issue after r267961. Handle special string case ...

Reviewed by: olce, kib
Fixes: af3b2549c4ba Pull in r267961 and r267973 again ...
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D48898

(cherry picked from commit 7d4c0fac8c7db9c5741ba98a8b3ce3c43feb1cf4)
(cherry picked from commit 04f360b782205c799b9302ba8bd810f7a303017f)