powerpc/powernv/npu: Fix deadlock in mmio_invalidate()

When sending TLB invalidates to the NPU we need to send extra flushes due
to a hardware issue. The original implementation would lock the all the
ATSD MMIO registers sequentially before unlocking and relocking each of
them sequentially to do the extra flush.

This introduced a deadlock as it is possible for one thread to hold one
ATSD register whilst waiting for another register to be freed while the
other thread is holding that register waiting for the one in the first
thread to be freed.

For example if there are two threads and two ATSD registers:

  Thread A	Thread B
  ----------------------
  Acquire 1
  Acquire 2
  Release 1	Acquire 1
  Wait 1	Wait 2

Both threads will be stuck waiting to acquire a register resulting in an
RCU stall warning or soft lockup.

This patch solves the deadlock by refactoring the code to ensure registers
are not released between flushes and to ensure all registers are either
acquired or released together and in order.

Fixes: bbd5ff50

 ("powerpc/powernv/npu-dma: Add explicit flush when sending an ATSD")
Signed-off-by: Alistair Popple <alistair@popple.id.au>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>