netmap: harden nmreq option size and overflow checks (defensive) #1866
      
        
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Summary: Enforce exact per‑option body size equality; add explicit overflow guards for size arithmetic; reject invalid lengths with EINVAL. Defensive hardening; no API break.
Test Plan: malformed NIOCCTRL (oversized nro_size, invalid types) → EINVAL/ENOTTY/ENXIO; no panic; valid flows unchanged.
Risk: Low; only invalid inputs are rejected earlier.