arm64: uapi: Provide correct __BITS_PER_LONG for the compat vDSO

The generic vDSO library uses the UAPI headers. On arm64 __BITS_PER_LONG is
always '64' even when used from the compat vDSO. In that case __GENMASK()
does an illegal bitshift, invoking undefined behaviour.

Change __BITS_PER_LONG to also work when used from the comapt vDSO.
To not confuse real userspace, only do this when building the kernel.

Reported-by: John Stultz <[email protected]>
Closes: https://lore.kernel.org/lkml/CANDhNCqvKOc9JgphQwr0eDyJiyG4oLFS9R8rSFvU0fpurrJFDg@mail.gmail.com/
Fixes: cd3557a ("vdso/gettimeofday: Add support for auxiliary clocks")
Signed-off-by: Thomas Weißschuh <[email protected]>
Acked-by: Arnd Bergmann <[email protected]>
Tested-by: John Stultz <[email protected]>
Link: https://lore.kernel.org/r/20250821-vdso-arm64-compat-bitsperlong-v1-1-700bcabe7732@linutronix.de
Signed-off-by: Catalin Marinas <[email protected]>

Author: t-8ch

arch/arm64/include/uapi/asm/bitsperlong.h

@@ -17,7 +17,12 @@
 #ifndef __ASM_BITSPERLONG_H
 #define __ASM_BITSPERLONG_H
 
+#if defined(__KERNEL__) && !defined(__aarch64__)
+/* Used by the compat vDSO */
+#define __BITS_PER_LONG 32
+#else
 #define __BITS_PER_LONG 64
+#endif
 
 #include <asm-generic/bitsperlong.h>