dns: better error handling when parsing names

The DNS name parser will error out with an error even if the
error is incomplete. Instead of manually generating errors,
use '?' to let the nom error ripple up the error handling chain.

The reason this wasn't done in the first place is this code
predates the ? operator, or we were not aware of it at the time.

This prevents the case where probing fails when there is enough data to
parse the header, but not enough to complete name parser. In such a case
a parse error is returned (instead of incomplete) resulting in the
payload not being detected as DNS.

Ticket #5034

(cherry picked from commit 0623ada24d)

rust/src/dns/parser.rs

@@ -20,7 +20,6 @@
 use nom::IResult;
 use nom::combinator::rest;
 use nom::error::ErrorKind;
-use nom::multi::length_data;
 use nom::number::streaming::{be_u8, be_u16, be_u32};
 use nom;
 use crate::dns::dns::*;
@@ -71,36 +70,21 @@ pub fn dns_parse_name<'a, 'b>(start: &'b [u8],
             pos = &pos[1..];
             break;
         } else if len & 0b1100_0000 == 0 {
-            match length_data(be_u8)(pos) as IResult<&[u8],_> {
-                Ok((rem, label)) => {
-                    if name.len() > 0 {
-                        name.push('.' as u8);
-                    }
-                    name.extend(label);
-                    pos = rem;
-                }
-                _ => {
-                    return Err(nom::Err::Error(
-                        error_position!(pos, ErrorKind::OctDigit)));
-                }
+            let (rem, label) = nom::length_data!(pos, be_u8)?;
+            if name.len() > 0 {
+                name.push('.' as u8);
             }
+            name.extend(label);
+            pos = rem;
         } else if len & 0b1100_0000 == 0b1100_0000 {
-            match be_u16(pos) as IResult<&[u8],_> {
-                Ok((rem, leader)) => {
-                    let offset = usize::from(leader) & 0x3fff;
-                    if offset > message.len() {
-                        return Err(nom::Err::Error(
-                            error_position!(pos, ErrorKind::OctDigit)));
-                    }
-                    pos = &message[offset..];
-                    if pivot == start {
-                        pivot = rem;
-                    }
-                }
-                _ => {
-                    return Err(nom::Err::Error(
-                        error_position!(pos, ErrorKind::OctDigit)));
-                }
+            let (rem, leader) = be_u16(pos)?;
+            let offset = usize::from(leader) & 0x3fff;
+            if offset > message.len() {
+                return Err(nom::Err::Error(error_position!(pos, nom::error::ErrorKind::OctDigit)));
+            }
+            pos = &message[offset..];
+            if pivot == start {
+                pivot = rem;
             }
         } else {
             return Err(nom::Err::Error(