libcontainer: fix bad conversion from syscall.Errno to error
libcontainer: fix bad conversion from syscall.Errno to error

The correct way to do that conversion according to
https://pkg.go.dev/syscall#Errno is:

```
err = nil
if errno != 0 {
	err = errno
}
```

In this case the error check will always report a false positive in
unix.RawSyscall(unix.SYS_SECCOMP, ...), probably nobody has faced this
problem because the code takes the other path in most of the cases.

Fixes: 7a8d7162f9d7 ("seccomp: prepend -ENOSYS stub to all filters")

Signed-off-by: Mauricio Vásquez <95f407c1ffa7689a0a3f2f21b1bf582fe8359351@kinvolk.io>

kolyshkin/runc,estesp/runc,opencontainers/runc,dqminh/runc,dqminh/runc,kolyshkin/runc,kolyshkin/runc,rhatdan/runc,rhatdan/runc,estesp/runc,kolyshkin/runc,opencontainers/runc,opencontainers/runc,dqminh/runc,dqminh/runc,rhatdan/runc,estesp/runc,opencontainers/runc,rhatdan/runc