signal: update outdated comment for removed freezable_schedule()

signal: update outdated comment for removed freezable_schedule()

The function freezable_schedule() was removed in commit
f5d39b020809 ("freezer,sched: Rewrite core freezer logic"), which
rewrote the freezer to use a dedicated TASK_FROZEN state instead.

do_signal_stop() and ptrace_stop() no longer call
freezable_schedule(); they now set TASK_STOPPED/TASK_TRACED and the
freezer handles those states directly via TASK_FROZEN.  Update the
comment to reflect the current mechanism.

Assisted-by: unnamed:deepseek-v3.2 coccinelle
Signed-off-by: Kexin Sun <kexinsun@smail.nju.edu.cn>
Link: https://patch.msgid.link/20260321105927.7979-1-kexinsun@smail.nju.edu.cn
Signed-off-by: Christian Brauner <brauner@kernel.org>