block: add missed aio_context_acquire into release_drive

block: add missed aio_context_acquire into release_drive

Recently we expirience hang with iothreads enabled with the following
call trace:
Thread 1 (Thread 0x7fa95efebc80 (LWP 177117)):
0  ppoll () from /lib64/libc.so.6
2  qemu_poll_ns () at qemu-timer.c:313
3  aio_poll () at aio-posix.c:457
4  bdrv_flush () at block/io.c:2641
5  bdrv_close () at block.c:2143
6  bdrv_delete () at block.c:2352
7  bdrv_unref () at block.c:3429
8  blk_remove_bs () at block/block-backend.c:427
9  blk_delete () at block/block-backend.c:178
10 blk_unref () at block/block-backend.c:226
11 object_property_del_all () at qom/object.c:399
12 object_finalize () at qom/object.c:461
13 object_unref () at qom/object.c:898
14 object_property_del_child () at qom/object.c:422
15 qmp_marshal_device_del () at qmp-marshal.c:1145
16 handle_qmp_command () at /usr/src/debug/qemu-2.6.0/monitor.c:3929

Technically bdrv_flush() stucks in
    while (rwco.ret == NOT_DONE) {
        aio_poll(aio_context, true);
    }
but rwco.ret is equal to 0 thus we have missed wakeup. Code investigation
reveals that we do not have performed aio_context_acquire() on this call
stack.

This patch adds missed lock.

Signed-off-by: Denis V. Lunev <den@openvz.org>
CC: Kevin Wolf <kwolf@redhat.com>
CC: Max Reitz <mreitz@redhat.com>
CC: Eric Blake <eblake@redhat.com>
CC: Markus Armbruster <armbru@redhat.com>
Message-id: 1490717566-25516-1-git-send-email-den@openvz.org
Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>