kbuild: Enable -Wenum-conversion by default

This diagnostic checks whether there is a type mismatch when
converting enums (assign an enum of type A to an enum of type B, for
example) and it caught a legit issue recently. The reason it didn't show
is because that warning is enabled only with -Wextra with GCC. Clang,
however, enables it by default.

GCC folks were considering enabling it by default but it was too noisy
back then:

  https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78736

Now that due to clang all those warnings have been fixed, enable it with
GCC too.

allmodconfig tests done with: x86, arm{,64}, powerpc{,64}, riscv
crossbuilds.
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

Makefile

@@ -1090,6 +1090,9 @@ KBUILD_CFLAGS   += $(call cc-option,-Werror=incompatible-pointer-types)
 # Require designated initializers for all marked structures
 KBUILD_CFLAGS   += $(call cc-option,-Werror=designated-init)
 
+# Warn if there is an enum types mismatch
+KBUILD_CFLAGS	+= $(call cc-option,-Wenum-conversion)
+
 # change __FILE__ to the relative path from the srctree
 KBUILD_CPPFLAGS += $(call cc-option,-fmacro-prefix-map=$(srctree)/=)