nvme: Skip checking heads without namespaces

nvme: Skip checking heads without namespaces

BugLink: http://bugs.launchpad.net/bugs/1769723
commit 2079699c10c8c60a9572540c2f77d045abf036eb upstream.

If a task is holding a reference to a namespace on a removed controller,
the head will not be released. If the same controller is added again
later, its namespaces may not be successfully added. Instead, the user
will see kernel message "Duplicate IDs for nsid <X>".

This patch fixes that by skipping heads that don't have namespaces when
considering if a new namespace is safe to add.

Reported-by: Alex Gagniuc <Alex_Gagniuc@Dellteam.com>
Cc: stable@vger.kernel.org
Signed-off-by: Keith Busch <keith.busch@intel.com>
Reviewed-by: Max Gurtovoy <maxg@mellanox.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>