block: move drain outside of bdrv_set_backing_hd_drained()

block: move drain outside of bdrv_set_backing_hd_drained()

This is part of resolving the deadlock mentioned in commit "block:
move draining out of bdrv_change_aio_context() and mark GRAPH_RDLOCK".

The function bdrv_set_backing_hd_drained() holds the graph lock, so it
is not allowed to drain. It is called by:
1. bdrv_set_backing_hd(), where a drained section is introduced,
   replacing the previously present bs-specific drains.
2. stream_prepare(), where a drained section is introduced replacing
   the previously present bs-specific drains.

The drain_bs variable in bdrv_set_backing_hd_drained() is now
superfluous and thus dropped.

Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
Message-ID: <20250530151125.955508-12-f.ebner@proxmox.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>