time: Prevent time64_to_tm() day truncation on 32-bit

time64_to_tm() accepts a 64-bit seconds value, but stores the quotient in
long. On a 32-bit kernel the day count therefore wraps when it reaches
2^31, even though the corresponding year remains representable in
struct tm.

This is reachable when formatting externally supplied timestamps. For
example, NILFS recovery copies the little-endian 64-bit ss_create field
from an on-disk segment summary into time64_t. Its sysfs attributes then
print that value with %ptTs, whose formatter calls time64_to_tm(). A
corrupted image can consequently produce an architecture-dependent printed
date.

Keep the day quotient in s64 while leaving the seconds-within-day remainder
as long. Use div_s64_rem() for the weekday calculation so 32-bit builds do
not require compiler runtime division helpers.

Fixes: e6c2682a1da3 ("time: Add time64_to_tm()")
Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Assisted-by: LLM
Reviewed-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Link: https://patch.msgid.link/20260925165702.51677-2-kmehltretter@gmail.com

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