selftests/bpf: Trace bpf_local_storage_update to debug flaky local storage tests

task_local_storage/sys_enter_exit and cgrp_local_storage/
cgroup_iter_sleepable occasionally fail in CI possibly because
bpf_{task,cgrp}_storage_get() returns NULL. Add a fexit probe on
bpf_local_storage_update() to capture the actual error code when this
happens. It will allow us to tell if it is trylock failure in
kmalloc_nolock(), timeout/deadlock in rqspinlock or something else.

Signed-off-by: Amery Hung <ameryhung@gmail.com>
Acked-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/bpf/20260417233631.1443199-1-ameryhung@gmail.com
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>