bcachefs: set_worker_desc() for delete_dead_snapshots

this is long running - help users see what's going on
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/snapshot.c

@@ -1680,6 +1680,8 @@ void bch2_delete_dead_snapshots_work(struct work_struct *work)
 {
 	struct bch_fs *c = container_of(work, struct bch_fs, snapshot_delete_work);
 
+	set_worker_desc("bcachefs-delete-dead-snapshots/%s", c->name);
+
 	bch2_delete_dead_snapshots(c);
 	bch2_write_ref_put(c, BCH_WRITE_REF_delete_dead_snapshots);
 }