[ebpf] Explicitly specify type with EbpfHelperImpl

An upcoming Rust toolchain change
rust-lang/rust#148602 can no longer infer
these functions implement the method of an associated type.

Change-Id: I04a13aae47036035601d777c8d1470d0d3f1dcde
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1444593
Reviewed-by: Adam Perry <adamperry@google.com>
Fuchsia-Auto-Submit: Erick Tryzelaar <etryzelaar@google.com>
Commit-Queue: Erick Tryzelaar <etryzelaar@google.com>

Author: erickt

src/starnix/lib/ebpf/src/conformance.rs

@@ -613,7 +613,7 @@ pub mod test {
                     return_value: Type::UNKNOWN_SCALAR,
                     invalidate_array_bounds: false,
                 },
-                EbpfHelperImpl(write_only),
+                EbpfHelperImpl::<TestEbpfProgramContext>(write_only),
             ),
             (
                 103,
@@ -629,7 +629,7 @@ pub mod test {
                     })),
                     invalidate_array_bounds: false,
                 },
-                EbpfHelperImpl(malloc),
+                EbpfHelperImpl::<TestEbpfProgramContext>(malloc),
             ),
             (
                 104,
@@ -638,7 +638,7 @@ pub mod test {
                     return_value: Type::UNKNOWN_SCALAR,
                     invalidate_array_bounds: false,
                 },
-                EbpfHelperImpl(free),
+                EbpfHelperImpl::<TestEbpfProgramContext>(free),
             ),
         ]
     });