compat: linux: use appropriate variables for copying out old timers

We copyout &l_oval but do the conversions into &l_val, leaving us with
stack garbage. A build with an LLVM21 cross-toolchain seems to catch
this.

Reported by: Florian Limberger <flo purplekraken com>
Reviewed by: markj
Fixes: a1fd2911ddb06 ("linux(4): Implement timer_settime64 syscall.")
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D52985

(cherry picked from commit 541a98d7e28a8e4697ac2fa78dd4c4203c2c3a9c)