block/snapshot: move drain outside of read-locked bdrv_snapshot_delete()

block/snapshot: move drain outside of read-locked bdrv_snapshot_delete()

This is in preparation to mark bdrv_drained_begin() as GRAPH_UNLOCKED.

More granular draining is not trivially possible, because
bdrv_snapshot_delete() can recursively call itself.

The return value of bdrv_all_delete_snapshot() changes from -1 to
-errno propagated from failed sub-calls. This is fine for the existing
callers of bdrv_all_delete_snapshot().

Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Message-ID: <20250530151125.955508-4-f.ebner@proxmox.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>