block-backend: Decrease in_flight only after callback

Request callbacks can do pretty much anything, including operations that
will yield from the coroutine (such as draining the backend). In that
case, a decreased in_flight would be visible to other code and could
lead to a drain completing while the callback hasn't actually completed
yet.

Note that reordering these operations forbids calling drain directly
inside an AIO callback. As Paolo explains, indirectly calling it is
okay:

- Calling it through a coroutine is okay, because then
  bdrv_drained_begin() goes through bdrv_co_yield_to_drain() and you
  have in_flight=2 when bdrv_co_yield_to_drain() yields, then soon
  in_flight=1 when the aio_co_wake() in the AIO callback completes, then
  in_flight=0 after the bottom half starts.

- Calling it through a bottom half would be okay too, as long as the AIO
  callback remembers to do inc_in_flight/dec_in_flight just like
  bdrv_co_yield_to_drain() and bdrv_co_drain_bh_cb() do

A few more important cases that come to mind:

- A coroutine that yields because of I/O is okay, with a sequence
  similar to bdrv_co_yield_to_drain().

- A coroutine that yields with no I/O pending will correctly decrease
  in_flight to zero before yielding.

- Calling more AIO from the callback won't overflow the counter just
  because of mutual recursion, because AIO functions always yield at
  least once before invoking the callback.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>

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