block: don't acquire AioContext lock in bdrv_drain_all()

block: don't acquire AioContext lock in bdrv_drain_all()

There is no need for the AioContext lock in bdrv_drain_all() because
nothing in AIO_WAIT_WHILE() needs the lock and the condition is atomic.

AIO_WAIT_WHILE_UNLOCKED() has no use for the AioContext parameter other
than performing a check that is nowadays already done by the
GLOBAL_STATE_CODE()/IO_CODE() macros. Set the ctx argument to NULL here
to help us keep track of all converted callers. Eventually all callers
will have been converted and then the argument can be dropped entirely.

Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20230309190855.414275-2-stefanha@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Wilfred Mallawa <wilfred.mallawa@wdc.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>