netmap: Fix TOCTOU vulnerability in nmreq_copyin

The total size of the user-provided nmreq was first computed and then
trusted during the copyin. This might lead to kernel memory corruption
and escape from jails/containers.

Reported by: Lucas Leong (@_wmliang_) of Trend Micro Zero Day Initiative
Security: CVE-2022-23084
MFC after: 3 days