block/vvfat: child_vvfat_qcow: add .get_parent_aio_context, fix crash

Commit 3ca1f322
"block: BdrvChildClass: add .get_parent_aio_context handler" introduced
new handler and commit 228ca37e
"block: drop ctx argument from bdrv_root_attach_child" made a generic
use of it. But 3ca1f322 didn't update
child_vvfat_qcow. Fix that.

Before that fix the command

./build/qemu-system-x86_64 -usb -device usb-storage,drive=fat16 \
  -drive file=fat:rw:fat-type=16:"<path of a host folder>",id=fat16,format=raw,if=none

crashes:

1  bdrv_child_get_parent_aio_context (c=0x559d62426d20)
    at ../block.c:1440
2  bdrv_attach_child_common
    (child_bs=0x559d62468190, child_name=0x559d606f9e3d "write-target",
     child_class=0x559d60c58d20 <child_vvfat_qcow>, child_role=3,
     perm=3, shared_perm=4, opaque=0x559d62445690,
     child=0x7ffc74c2acc8, tran=0x559d6246ddd0, errp=0x7ffc74c2ae60)
    at ../block.c:2795
3  bdrv_attach_child_noperm
    (parent_bs=0x559d62445690, child_bs=0x559d62468190,
     child_name=0x559d606f9e3d "write-target",
     child_class=0x559d60c58d20 <child_vvfat_qcow>, child_role=3,
     child=0x7ffc74c2acc8, tran=0x559d6246ddd0, errp=0x7ffc74c2ae60) at
    ../block.c:2855
4  bdrv_attach_child
    (parent_bs=0x559d62445690, child_bs=0x559d62468190,
     child_name=0x559d606f9e3d "write-target",
     child_class=0x559d60c58d20 <child_vvfat_qcow>, child_role=3,
     errp=0x7ffc74c2ae60) at ../block.c:2953
5  bdrv_open_child
    (filename=0x559d62464b80 "/var/tmp/vl.h3TIS4",
     options=0x559d6246ec20, bdref_key=0x559d606f9e3d "write-target",
     parent=0x559d62445690, child_class=0x559d60c58d20
     <child_vvfat_qcow>, child_role=3, allow_none=false,
     errp=0x7ffc74c2ae60) at ../block.c:3351
6  enable_write_target (bs=0x559d62445690, errp=0x7ffc74c2ae60) at
   ../block/vvfat.c:3176
7  vvfat_open (bs=0x559d62445690, options=0x559d6244adb0, flags=155650,
               errp=0x7ffc74c2ae60) at ../block/vvfat.c:1236
8  bdrv_open_driver (bs=0x559d62445690, drv=0x559d60d4f7e0
                     <bdrv_vvfat>, node_name=0x0,
                     options=0x559d6244adb0, open_flags=155650,
                     errp=0x7ffc74c2af70) at ../block.c:1557
9  bdrv_open_common (bs=0x559d62445690, file=0x0,
                     options=0x559d6244adb0, errp=0x7ffc74c2af70) at
...

(gdb) fr 1
 #1  0x0000559d603ea3bf in bdrv_child_get_parent_aio_context
     (c=0x559d62426d20) at ../block.c:1440
1440        return c->klass->get_parent_aio_context(c);
 (gdb) p c->klass
$1 = (const BdrvChildClass *) 0x559d60c58d20 <child_vvfat_qcow>
 (gdb) p c->klass->get_parent_aio_context
$2 = (AioContext *(*)(BdrvChild *)) 0x0

Fixes: 3ca1f322
Fixes: 228ca37e
Reported-by: John Arbuckle <programmingkidx@gmail.com>
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20210524101257.119377-2-vsementsov@virtuozzo.com>
Tested-by: John Arbuckle <programmingkidx@gmail.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>