fork: move the coredump and exec checks into create_io_thread()

Only a PF_USER_WORKER thread returns from get_signal() after a fatal
signal and can go on to create a thread. get_signal() sets PF_SIGNALED
before it dumps core and before it lets a PF_USER_WORKER thread return.
That flags sticks.

The exec'ing thread itself is never signaled. But it doesn't need the check
anyway as we moved cancellation of io_uring requests before de_thread().
create_io_thread() is the only way a signaled thread creates another
one. vhost_task_create() is reached from ioctls alone.

io_should_retry_thread() doesn't retry -EINTR, so io-wq callers give up
the same way they did when copy_process() refused.

Suggested-by: Oleg Nesterov <oleg@redhat.com>
Link: https://lore.kernel.org/r/aq0VLTyxNatFmLOK@redhat.com
Link: https://patch.msgid.link/20260921-work-coredump-fixes-v3-13-8e4adb1619e6@kernel.org
Acked-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

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