'struct nshead' and 'struct ns' have different lifetimes; the nshead
can (and occasionally will) stay around even if no namespaces are
connected, as it will only be finally removed once the last
opener is gone.
Once the system enters this state it becomes very hard to recover,
as the only way to remove a namespace is to disconnect the controller,
but the nshead is precisely _not_ related to a controller. This leads
to the very annoying behaviour that a subsequent 'nvme connect' call
will establish a _different_ nshead, _and_ the previous nshead will
stay around:

quoted

nvme list

/dev/nvme0n1     9329c4c4af01db2c     Linux \
     1          17.18  GB /  17.18  GB      4 KiB +  0 B   5.3.18-4

quoted

nvme disconnect -d /dev/nvme0
nvme list

/dev/nvme0n1                                \
     -1          0.00   B /   0.00   B      1   B +  0 B   n1

quoted

nvme connect
nvme list

/dev/nvme0n1     9329c4c4af01db2c     Linux \
     -1          0.00   B /   0.00   B      1   B +  0 B   5.3.18-4
/dev/nvme0n2     9329c4c4af01db2c     Linux \
     1          17.18  GB /  17.18  GB      4 KiB +  0 B   5.3.18-4

This patch fixes the situation by only removing the nshead from the
internal lists once it's being finally removed.

I'd say that we should remove the nshead exactly when the last
lower ns was removed? Why should it still occupy sysfs and ida
resources if it is going away soon?

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