nvme: fix the deadlock in nvme_update_formats

nvme_update_formats will invoke nvme_ns_remove under namespaces_mutext.
The will cause deadlock because nvme_ns_remove will also require
the namespaces_mutext. Fix it by getting the ns entries which should
be removed under namespaces_mutext and invoke nvme_ns_remove out of
namespaces_mutext.

Signed-off-by: Jianchao Wang <jianchao.w.wang@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Sagi Grimberg <sagi@grimberg.me>