nvmet: fix max_qid race between configfs and controller allocation

In the Linux kernel, the following vulnerability has been resolved:

nvmet: fix max_qid race between configfs and controller allocation

The function nvmet_subsys_attr_qid_max_store() can race against
nvmet_alloc_ctrl() when a subsystem's max_qid limit is modified.

Suppose max_qid is currently 64. If nvmet_alloc_ctrl() executes:
ctrl->sqs = kzalloc_objs(struct nvmet_sq *, subsys->max_qid + 1);
and at this exact point, a userspace process changes max_qid to 128,
nvmet_subsys_attr_qid_max_store() will set the new max_qid value. It
attempts to delete active controllers to force a reconnect, but the
new controller won't be deleted because it hasn't been added to the
subsys->ctrls list yet.

nvmet_alloc_ctrl() then proceeds and adds the new controller to the
subsys->ctrls list. Later, when nvmet_install_queue() is called, it
will see max_qid set to 128, but the memory allocated for sqs is only
sized for 64 entries. This results in a KASAN out-of-bounds warning
and potential memory corruptions.

Fix this by protecting the queue allocations and list insertion in
nvmet_alloc_ctrl() with down_read(&nvmet_config_sem). Because
nvmet_subsys_attr_qid_max_store() acquires down_write(&nvmet_config_sem)
to modify the attribute, this safely prevents the configfs writer from
modifying max_qid during controller creation.

Copy the max_qid from the subsystem to the controller's structure
during the allocation; ctrl->max_qid never changes as long as the
controller remains in LIVE state, so this will prevent similar race
conditions.