Fix syscall_info

According to the man page:

> The addr argument contains the size of the buffer pointed to by the
> data argument (i.e., sizeof(struct ptrace_syscall_info)).

We were setting the data argument, so this syscall was returning
garbage. This is easily reproduced and verified by looking at the `op`
value, which should range from 0 to 3. The PR associated with this
commit contains a sample program to demonstrate this.

The fix is done in the `ptrace_get_data` helper to avoid duplicating its
implementation just for `syscall_info`.

Of all the other callers, all but one are documented as ignoring `addr`.
The other one (`PTRACE_GETREGS`) is documented as ignoring `addr`
_except on SPARC systems_, on which `addr` and `data` are reversed.
However, this is already not respected by `nix`, so this changes is not
disruptive in this regard.

There should be no performance concerns as we are replacing one constant
with another.

Author: cbarrete

src/sys/ptrace/linux.rs

@@ -504,7 +504,7 @@ fn ptrace_get_data<T>(request: Request, pid: Pid) -> Result<T> {
         libc::ptrace(
             request as RequestType,
             libc::pid_t::from(pid),
-            ptr::null_mut::<T>(),
+            size_of::<T>(),
             data.as_mut_ptr(),
         )
     };