[PATCHv2-4.5 04/10] NVMe: Fix namespace removal deadlock

So, this one is the thing I don't like.  The real issue here is the
broad scope of namespaces_mutex.  I think we should have a small
scope lock protecting just the list manipulations, which is not
held over I/O, and the issue would sort itself out.

Note that we already have a high level serialization for scanning
by always executing it in the scan_work work_item, so there is no
need for the high level serialization anyway.