bpf: Simplify bpf_timer_cancel()

bpf: Simplify bpf_timer_cancel()

Remove lock from the bpf_timer_cancel() helper. The lock does not
protect from concurrent modification of the bpf_async_cb data fields as
those are modified in the callback without locking.

Use guard(rcu)() instead of pair of explicit lock()/unlock().

Acked-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Mykyta Yatsenko <yatsenko@meta.com>
Link: https://lore.kernel.org/r/20260120-timer_nolock-v6-4-670ffdd787b4@meta.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>