rust: globally deny unsafe_op_in_unsafe_fn

This prevents new ones from getting in.

Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Makefile

@@ -516,7 +516,7 @@ KBUILD_RUSTCFLAGS := --emit=dep-info,obj,metadata --edition=2018 \
 		     -Cpanic=abort -Cembed-bitcode=n -Clto=n -Crpath=n \
 		     -Cforce-unwind-tables=n -Ccodegen-units=1 \
 		     -Zbinary_dep_depinfo=y -Zsymbol-mangling-version=v0 \
-		     -W unsafe_op_in_unsafe_fn
+		     -D unsafe_op_in_unsafe_fn
 KBUILD_AFLAGS_KERNEL :=
 KBUILD_CFLAGS_KERNEL :=
 KBUILD_RUSTCFLAGS_KERNEL :=