y2038: uapi: Use 64-bit __kernel_old_timespec::tv_nsec on x32

'struct __kernel_old_timespec' represents the 'native' time ABI of the
kernel. On 32-bit systems it uses 32-bit fields and on 64-bit systems
it uses 64-bit fields.

However the x86 x32 ABI uses the 64-bit time ABI natively.  This is
correctly handled for the 'tv_sec' fields, through the typedefs of
'__kernel_old_time_t' -> '__kernel_long_t' -> 'long long'. The same
treatment was missed for 'tv_nsec'.

In practice this might not make much of a difference as the value of
'tv_nsec' will always fit into 32 bits and the missing bits fall
into the padding of the structure.

When introspecting the structure however, a difference can be observed.

Switch to 64-bit tv_nsec on x32.  No other architectures or ABIs are
affected.

While this could be interpreted as violating the POSIX requirement of
'timespec::tv_nsec' being 'long':

  * __kernel_old_timespec is not actually the POSIX timespec type

  * the requirement is gone in newer versions of POSIX

  * this matches glibc

Fixes: 94c467ddb2 ("y2038: add __kernel_old_timespec and __kernel_old_time_t")
Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Link: https://patch.msgid.link/20260504-timespec-x32-v2-1-0739c9047fc4@linutronix.de

include/uapi/linux/time_types.h

@@ -30,7 +30,7 @@ struct __kernel_old_timeval {
 
 struct __kernel_old_timespec {
 	__kernel_old_time_t	tv_sec;		/* seconds */
-	long			tv_nsec;	/* nanoseconds */
+	__kernel_long_t		tv_nsec;	/* nanoseconds */
 };
 
 struct __kernel_old_itimerval {