fuse: update fuse_conn_init() and separate out fuse_conn_kill()

Update fuse_conn_init() such that it doesn't take @sb and move bdi
registration into a separate function.  Also separate out
fuse_conn_kill() from fuse_put_super().

These will be used to implement cuse.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
diff --git a/fs/fuse/dev.c b/fs/fuse/dev.c
index ba76b68..368189f 100644
--- a/fs/fuse/dev.c
+++ b/fs/fuse/dev.c
@@ -282,7 +282,7 @@
 			wake_up_all(&fc->blocked_waitq);
 		}
 		if (fc->num_background == FUSE_CONGESTION_THRESHOLD &&
-		    fc->connected) {
+		    fc->connected && fc->bdi_initialized) {
 			clear_bdi_congested(&fc->bdi, READ);
 			clear_bdi_congested(&fc->bdi, WRITE);
 		}
@@ -408,7 +408,8 @@
 	fc->num_background++;
 	if (fc->num_background == FUSE_MAX_BACKGROUND)
 		fc->blocked = 1;
-	if (fc->num_background == FUSE_CONGESTION_THRESHOLD) {
+	if (fc->num_background == FUSE_CONGESTION_THRESHOLD &&
+	    fc->bdi_initialized) {
 		set_bdi_congested(&fc->bdi, READ);
 		set_bdi_congested(&fc->bdi, WRITE);
 	}