nvme: ns_head vs namespace mismatch fixes

[Anton Eidelman <anton@lightbitslabs.com>]

Please, apply the following two upstream commits (attached)
(in this order):
    d567572906d9 nvme: unlink head after removing last namespace
    ac262508daa8 nvme: release namespace head reference on error

TO: v5.4, v5.5, v5.6, v5.7
These commits are present in v5.8
and apply cleanly to the above.

Reason:
These fix a potential crash or malfunction
when an nvme namespace is deleted
and then a new namespace with the same nsid is created
before the old ns_head for this nsid is gone.

The first commit prevents the new namespace
from being matched by nvme_init_ns_head()
with the old ns_head causing ID mismatch
and consequently a failure to initialize the new namespace.

The second commit prevents ns_head refcount imbalance
in case nvme_init_ns_head() detects ID mismatch,
and consequently a crash later.

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