libnetmap: fix extra indirection in nmreq_remove_option

Reviewed by: zlei, vmaffione
Obtained from: https://github.com/luigirizzo/netmap/commit/7d9177ed9a121e66bf4eaa0acb5d574e408297da
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D58151

(cherry picked from commit 141b6645204966a0e1ae10dd059c670e2a58e6e1)