softirq: Replace ksoftirqd with workqueues entirely

Ksoftirqd only remains to implement threaded IRQs. Convert it to
existing per-vector workqueues to avoid code duplication.

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Suggested-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Cc: Dmitry Safonov <dima@arista.com>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: David Miller <davem@davemloft.net>
Cc: Hannes Frederic Sowa <hannes@stressinduktion.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Levin Alexander <alexander.levin@verizon.com>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Radu Rendec <rrendec@arista.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Stanislaw Gruszka <sgruszka@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Wanpeng Li <wanpeng.li@hotmail.com>
Cc: Mauro Carvalho Chehab <mchehab@s-opensource.com>