x86/fpu: Make WARN_ON_FPU() more robust in the !CONFIG_X86_DEBUG_FPU case

Make sure the WARN_ON_FPU() macro consumes the macro argument,
to avoid 'unused variable' build warnings if the only use of
a variable is in debugging code.

Cc: Andy Lutomirski <[e-mail]>
Cc: Bobby Powers <[e-mail]>
Cc: Borislav Petkov <[e-mail]>
Cc: Dave Hansen <[e-mail]>
Cc: Fenghua Yu <[e-mail]>
Cc: H. Peter Anvin <[e-mail]>
Cc: Linus Torvalds <[e-mail]>
Cc: Oleg Nesterov <[e-mail]>
Cc: Thomas Gleixner <[e-mail]>
Signed-off-by: Ingo Molnar <[e-mail]>

arch/x86/include/asm/fpu/internal.h

···
 #ifdef CONFIG_X86_DEBUG_FPU
 # define WARN_ON_FPU(x) WARN_ON_ONCE(x)
 #else
-# define WARN_ON_FPU(x) ({ 0; })
+# define WARN_ON_FPU(x) ({ (void)(x); 0; })
 #endif
 
 /*