Fix build with nodevice netmap

iru_init() was declared and used outside the DEV_NETMAP
conditional blocks, but was implemented inside one. Move the
implementation out of the DEV_NETMAP block to allow building with
netmap disabled.

Reported by: Andrew Turner <andrew@fubar.geek.nz>
Reviewed by: sbruno
Approved by: sbruno (mentor)
Sponsored by: Limelight Networks
Differential Revision: https://reviews.freebsd.org/D12842