so_reuseport_lb_test: correct setsockopt parameter size

Change type of variable used in setsocketopt so correct size of
option is passed.

Test failure was identified when running the test on PowerPC64,
and the following error message was seen:

"bind () failed: Address already in use"

Submitted by: Fernando Valle <fernando.valle@eldorado.org.br>
Reviewed by: melifaro, adalava
Approved by: jhibbits (mentor)
Sponsored by: Eldorado Research Institute (eldorado.org.br)
Differential Revision: https://reviews.freebsd.org/D24164