selftests/bpf: Update comments find_equal_scalars->sync_linked_regs

find_equal_scalars() is renamed to sync_linked_regs(),
this commit updates existing references in the selftests comments.

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20240718202357.1746514-5-eddyz87@gmail.com