uoa: fix build on kernels that renamed from_timer() to timer_container_of()

[pubyun]

Problem

Building the uoa kernel module on AlmaLinux 9.8 (kernel 5.14.0-687) fails:

uoa.c:498:26: error: implicit declaration of function 'from_timer'; did you mean 'mod_timer'? [-Werror=implicit-function-declaration]
  498 |     struct uoa_map *um = from_timer(um, timer, timer);

Upstream commit (Linux v6.16) renamed the from_timer() macro to timer_container_of(). RHEL/AlmaLinux backport this rename into their 5.14.0-based kernels, so the kernel version number stays at 5.14 while from_timer() is already gone. A LINUX_VERSION_CODE based guard cannot detect this case reliably — a >= KERNEL_VERSION(6,16,0) check would miss the backported 5.14.0-687, which is exactly what triggers the build failure.

Fix

Detect the API by macro presence instead of kernel version, and alias from_timer() to timer_container_of() only when the new name exists and the old one does not:

#if !defined(from_timer) && defined(timer_container_of)
#define from_timer(var, callback_timer, timer_fieldname) \
    timer_container_of(var, callback_timer, timer_fieldname)
#endif

The shim is placed right after #include <linux/timer.h>, so the timer helpers are already visible to the defined() checks.

Behavior across kernels:

• 4.15 – 6.15: from_timer is defined natively, the shim is skipped.
• 6.16+ and backported 5.14.0 kernels (e.g. AlmaLinux 9.8): from_timer is gone, timer_container_of exists, the shim maps the old name to the new one.
• < 4.15: neither macro exists; the shim is skipped, and the existing #if LINUX_VERSION_CODE >= KERNEL_VERSION(4,15,0) guard at the use site keeps the legacy unsigned long timer-callback path. No effect.

Testing

Verified to build cleanly on AlmaLinux 9.8 (5.14.0-687.5.4.el9_8.x86_64). The change is a compile-time macro shim with no runtime impact.

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