x/sys/unix: missing Linux system call name_to_handle_at

[bradfitz]

I needed to call the Linux name_to_handle_at (SYS_NAME_TO_HANDLE_AT = 303) system call for a test but it's not in x/sys/unix.

Likewise, looks like we need a type for struct file_handle *.

@tklauser, interested in adding? :)

[bradfitz]

Actually, the C struct isn't very mappable to Go types:

           struct file_handle {      
               unsigned int  handle_bytes;   /* Size of f_handle [in, out] */ 
               int           handle_type;    /* Handle type [out] */
               unsigned char f_handle[0];    /* File identifier (sized by    
                                                caller) [out] */
           };

So we'd likely need some higher-level Go wrapper around the system calls that dealt with the unsafety of making such a struct for the kernel.

[cuonglm]

@bradfitz duplicate of #30537

[bradfitz]

@cuonglm, whoops, thanks.