rust: fix off-by-one line number in rustdoc tests

When the `#![allow]` line was added, the doctest line number anchor
isn't updated which causes the line number printed in kunit test to be
off-by-one.

Fixes: ab844cf ("rust: allow `unreachable_pub` for doctests")
Signed-off-by: Gary Guo <gary@garyguo.net>
Reviewed-by: David Gow <davidgow@google.com>
Link: https://patch.msgid.link/20251211182208.2791025-1-gary@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: nbdd0121

scripts/rustdoc_test_gen.rs

@@ -206,7 +206,7 @@ pub extern "C" fn {kunit_name}(__kunit_test: *mut ::kernel::bindings::kunit) {{
 
     /// The anchor where the test code body starts.
     #[allow(unused)]
-    static __DOCTEST_ANCHOR: i32 = ::core::line!() as i32 + {body_offset} + 1;
+    static __DOCTEST_ANCHOR: i32 = ::core::line!() as i32 + {body_offset} + 2;
     {{
         #![allow(unreachable_pub, clippy::disallowed_names)]
         {body}