cmd/tailscaled,net/tstun: fix data race on start-up in TUN mode

Fixes #7894

Change-Id: Ice3f8019405714dd69d02bc07694f3872bb598b8

Co-authored-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Signed-off-by: Maisem Ali <maisem@tailscale.com>
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