coredump: simplify zap_process()

coredump: simplify zap_process()

After commit 0258b5fd7c71 ("coredump: Limit coredumps to a single thread
group") zap_process() doesn't need the "task_struct *start" arg,
zap_threads() can pass "signal_struct *signal" instead.

This simplifies the code and allows to use __for_each_thread() which
is slightly more efficient.

Link: https://lkml.kernel.org/r/20240625140311.GA20787@redhat.com
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>