create-diff-object: register patchable child functions

[benjamindonnachie]

Summary

• register changed compiler-generated child functions when they have their own profiling entry
• continue carrying children without a profiling entry through their changed parent
• avoid registering a child separately when a changed ancestor already carries its replacement code

Problem

GCC split snd_timer_interrupt() into an unchanged wrapper and a changed
snd_timer_interrupt.part.0. The child had its own __fentry__, so
kpatch_compare_sections() correctly avoided promoting the unchanged parent.

Later, kpatch_create_patches_sections() unconditionally skipped every symbol
with sym->parent. The resulting livepatch module contained the changed child
text and its KLP relocations, but no .kpatch.funcs registration record. The
module loaded successfully while the changed execution path remained unpatched.

Validation

• make -C kpatch-build -j4 (with -Werror): pass
• make -C test/unit ARCHES=x86_64: pass
• full EL9 exact-kernel build for 5.14.0-687.48.1.el9_8.x86_64: pass
• module metadata increased from 26 to 27 function records and included snd_timer_interrupt.part.0
• privileged runtime load: enabled=1, transition=0
• livepatch sysfs registered both snd_timer_interrupt.part.0,1 and snd_timer_close_locked,1

The failure was discovered while auditing a cumulative security livepatch: the
source/CVE manifest claimed coverage and the module loaded normally, but sysfs
showed the missing child-function registration.

Closes #1525