drm/amdgpu/userq: refcount userqueues to avoid any race conditions

drm/amdgpu/userq: refcount userqueues to avoid any race conditions

To avoid race condition and avoid UAF cases, implement kref
based queues and protect the below operations using xa lock
a. Getting a queue from xarray
b. Increment/Decrement it's refcount

Every time some one want to access a queue, always get via
amdgpu_userq_get to make sure we have locks in place and get
the object if active.

A userqueue is destroyed on the last refcount is dropped which
typically would be via IOCTL or during fini.

v2: Add the missing drop in one the condition in the signal ioclt [Alex]

v3: remove the queue from the xarray first in the free queue ioctl path
    [Christian]

- Pass queue to the amdgpu_userq_put directly.
- make amdgpu_userq_put xa_lock free since we are doing put for each get
  only and final put is done via destroy and we remove the queue from xa
  with lock.
- use userq_put in fini too so cleanup is done fully.

v4: Use xa_erase directly rather than doing load and erase in free
    ioctl. Also remove some of the error logs which could be exploited
    by the user to flood the logs [Christian]

Signed-off-by: Sunil Khatri <sunil.khatri@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>