It would acquire the lock, calculate `nextState`, relase
the lock, then call `enterState` which would acquire the lock
again. There were obvious races there which could lead to
nil panics as seen in a test in a different repo.
```
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x2 addr=0x70 pc=0x1050f2c7c]
goroutine 42240 [running]:
tailscale.com/ipn/ipnlocal.(*LocalBackend).enterStateLockedOnEntry(0x14002154e00, 0x6)
tailscale.com/ipn/ipnlocal/local.go:3715 +0x30c
tailscale.com/ipn/ipnlocal.(*LocalBackend).enterState(0x14002154e00?, 0x14002e3a140?)
tailscale.com/ipn/ipnlocal/local.go:3663 +0x8c
tailscale.com/ipn/ipnlocal.(*LocalBackend).stateMachine(0x14001f5e280?)
tailscale.com/ipn/ipnlocal/local.go:3836 +0x2c
tailscale.com/ipn/ipnlocal.(*LocalBackend).setWgengineStatus(0x14002154e00, 0x14002e3a190, {0x0?, 0x0?})
tailscale.com/ipn/ipnlocal/local.go:1193 +0x4d0
tailscale.com/wgengine.(*userspaceEngine).RequestStatus(0x14005d90300)
tailscale.com/wgengine/userspace.go:1051 +0x80
tailscale.com/wgengine.NewUserspaceEngine.func2({0x14002e3a0a0, 0x2, 0x140025cce40?})
tailscale.com/wgengine/userspace.go:318 +0x1a0
tailscale.com/wgengine/magicsock.(*Conn).updateEndpoints(0x14002154700, {0x105c13eaf, 0xf})
tailscale.com/wgengine/magicsock/magicsock.go:531 +0x424
created by tailscale.com/wgengine/magicsock.(*Conn).ReSTUN in goroutine 42077
tailscale.com/wgengine/magicsock/magicsock.go:2142 +0x3a4
```
Updates tailscale/corp#14480
Signed-off-by: Maisem Ali <maisem@tailscale.com>
cb3b281e98