fortify: Replace open-coded __gnu_inline attribute

Replace open-coded gnu_inline attribute with the normal kernel
convention for attributes: __gnu_inline

Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Link: https://lore.kernel.org/r/20220208225350.1331628-2-keescook@chromium.org