nvmet: initialize discovery subsys after debugfs is initialized

During nvme target initialization discovery subsystem is initialized
before "nvmet" debugfs directory is created. This results in discovery
subsystem debugfs directory to be created in debugfs root directory.

nvmet_init() ->
  nvmet_init_discovery() ->
    nvmet_subsys_alloc() ->
      nvmet_debugfs_subsys_setup()

In other words, the codepath above is exeucted before nvmet_debugfs is
created. We get /sys/kernel/debug/nqn.2014-08.org.nvmexpress.discovery
instead of /sys/kernel/debug/nvmet/nqn.2014-08.org.nvmexpress.discovery.
Move nvmet_init_discovery() call after nvmet_init_debugfs() to fix it.

Fixes: 649fd414 ("nvmet: add debugfs support")
Signed-off-by: Mohamed Khalfella <mkhalfella@purestorage.com>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Reviewed-by: Hannes Reinecke <hare@kernel.org>
Reviewed-by: Daniel Wagner <dwagner@suse.de>
Signed-off-by: Christoph Hellwig <hch@lst.de>