nvme-mpath: fix last path removal during traffic

In case our last path is removed during traffic, we can end up requeueing
the bio(s) but never schedule the actual requeue work as upper layers
still have open handles on the mpath device node.

Fix this by scheduling requeue work if the namespace being removed is
the last path in the ns_head path list.

Fixes: 32acab31 ("nvme: implement multipath access to nvme subsystems")
Signed-off-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>