rxrpc: Fix bundle counting for exclusive connections

Fix rxrpc_unbundle_conn() to not drop the bundle usage count when cleaning
up an exclusive connection.

Based on the suggested fix from Hillf Danton.

Fixes: 245500d8

 ("rxrpc: Rewrite the client connection manager")
Reported-by:  <syzbot+d57aaf84dd8a550e6d91@syzkaller.appspotmail.com>
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Hillf Danton <hdanton@sina.com>