nvmet-loop: do not alloc admin tag set during reset

nvmet-loop: do not alloc admin tag set during reset

Currently, resetting a loopback controller unconditionally invokes
nvme_alloc_admin_tag_set() inside nvme_loop_configure_admin_queue().
Doing so drops the old queue and allocates a new one. Consequently,
this reverts the admin queue's timeout (q->rq_timeout) back to the
module default (NVME_ADMIN_TIMEOUT), completely wiping out any custom
timeout values the user may have configured via sysfs and potentially
racing against the sysfs nvme_admin_timeout_store() function
that may dereference the admin_q pointer during the RESETTING state.

Decouple the admin tag set lifecycle from the admin queue
configuration and destruction paths, which are executed during resets;
Specifically:

* Move nvme_alloc_admin_tag_set() into nvme_loop_create_ctrl() so it
  is only allocated once during the initial controller creation.

* Defer the destruction of the admin tag set to
  nvme_loop_delete_ctrl_host() and the terminal error-handling
  paths of nvme_loop_reset_ctrl_work() and
  nvme_loop_create_ctrl().

Reviewed-by: Daniel Wagner <dwagner@suse.de>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Hannes Reinecke <hare@kernel.org>
Signed-off-by: Maurizio Lombardi <mlombard@redhat.com>
Signed-off-by: Keith Busch <kbusch@kernel.org>