Kernel supported features are detected using debugfs.
However, if the filesystem is not mounted, `syz-fuzzer` panics without
providing any clues as to why.

```
2024/05/04 10:12:49 connecting to manager...
2024/05/04 10:12:49 fuzzer vm-1 connected
2024/05/04 10:12:49 checking machine...
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x6019a8]

goroutine 1 [running]:
main.main()
        /home/alessandro/go/src/syzkaller/syz-fuzzer/fuzzer.go:169 +0x958
debug1: client_input_channel_req: channel 0 rtype exit-status reply 0
debug1: channel 0: free: client-session, nchannels 2
debug1: channel 1: free: 127.0.0.1, nchannels 1
Transferred: sent 5180, received 6532 bytes, in 2.5 seconds
Bytes per second: sent 2097.7, received 2645.2
debug1: Exit status 2
```

This simple patch prevents `syz-fuzzer` from crashing and allows it to
terminate cleanly, while provides a possible cause why this issue is
occurring.

```
2024/05/04 10:15:14 connecting to manager...
2024/05/04 10:15:14 fuzzer vm-1 connected
2024/05/04 10:15:14 checking machine...
2024/05/04 10:15:14 SYZFATAL: The currently running kernel image seems not to support any required feature, have you forgotten to mount debugfs?
debug1: client_input_channel_req: channel 0 rtype exit-status reply 0
debug1: channel 0: free: client-session, nchannels 2
debug1: channel 1: free: 127.0.0.1, nchannels 1
Transferred: sent 5160, received 5016 bytes, in 2.4 seconds
Bytes per second: sent 2106.7, received 2047.9
debug1: Exit status 1
```

Signed-off-by: Alessandro Carminati <[email protected]>