block/file-posix: don't use functions calling AIO_WAIT_WHILE in worker threads

block/file-posix: don't use functions calling AIO_WAIT_WHILE in worker threads

When calling bdrv_getlength() in handle_aiocb_write_zeroes(), the
function creates a new coroutine and then waits that it finishes using
AIO_WAIT_WHILE.
The problem is that this function could also run in a worker thread,
that has a different AioContext from main loop and iothreads, therefore
in AIO_WAIT_WHILE we will have in_aio_context_home_thread(ctx) == false
and therefore
assert(qemu_get_current_aio_context() == qemu_get_aio_context());
in the else branch will fail, crashing QEMU.

Aside from that, bdrv_getlength() is wrong also conceptually, because
it reads the BDS graph from another thread and is not protected by
any lock.

Replace it with raw_co_getlength, that doesn't create a coroutine and
doesn't read the BDS graph.

Reported-by: Ninad Palsule <ninad@linux.vnet.ibm.com>
Suggested-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
Message-Id: <20230209154522.1164401-1-eesposit@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>