nvmet: fix ns enable/disable possible hang

When disabling an nvmet namespace, there is a period where the
subsys->lock is released, as the ns disable waits for backend IO to
complete, and the ns percpu ref to be properly killed. The original
intent was to avoid taking the subsystem lock for a prolong period as
other processes may need to acquire it (for example new incoming
connections).

However, it opens up a window where another process may come in and
enable the ns, (re)intiailizing the ns percpu_ref, causing the disable
sequence to hang.

Solve this by taking the global nvmet_config_sem over the entire configfs
enable/disable sequence.

Fixes: a07b4970 ("nvmet: add a generic NVMe target")
Signed-off-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Signed-off-by: Keith Busch <kbusch@kernel.org>