task_work: unconditionally run task_work from get_signal()

Song reported a boot regression in a kvm image with 5.11-rc, and bisected
it down to the below patch. Debugging this issue, turns out that the boot
stalled when a task is waiting on a pipe being released. As we no longer
run task_work from get_signal() unless it's queued with TWA_SIGNAL, the
task goes idle without running the task_work. This prevents ->release()
from being called on the pipe, which another boot task is waiting on.

For now, re-instate the unconditional task_work run from get_signal().
For 5.12, we'll collapse TWA_RESUME and TWA_SIGNAL, as it no longer
makes sense to have a distinction between the two. This will turn
task_work notification into a simple boolean, whether to notify or not.

Fixes: 98b89b649fce ("signal: kill JOBCTL_TASK_WORK")
Reported-by: Song Liu <[email protected]>
Tested-by: John Stultz <[email protected]>
Tested-by: Douglas Anderson <[email protected]>
Tested-by: Sedat Dilek <[email protected]> # LLVM/Clang version 11.0.1
Signed-off-by: Jens Axboe <[email protected]>

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