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