drm/amdgpu: check amdgpu_vm_bo_find() result in GET_MAPPING_INFO

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

drm/amdgpu: check amdgpu_vm_bo_find() result in GET_MAPPING_INFO

The AMDGPU_GEM_OP_GET_MAPPING_INFO path of amdgpu_gem_op_ioctl() looks
up the bo_va for the buffer object in the caller's VM via
amdgpu_vm_bo_find(), but uses the returned pointer without checking it.

amdgpu_vm_bo_find() returns NULL when the BO has no bo_va in that VM,
which is the normal case for a BO that has never been mapped. The result
is fed straight into amdgpu_vm_bo_va_for_each_valid_mapping(), which
expands to list_for_each_entry(mapping, &(bo_va)->valids, list) and
dereferences bo_va, causing a NULL pointer dereference.

This is reachable by any process able to issue the ioctl (render group)
simply by requesting mapping info for an unmapped BO.

Return -ENOENT when no bo_va is found, jumping to out_exec so the
drm_exec context and GEM object reference are released.

(cherry picked from commit 528b19377affc1cc7362a70a254c1dda793595f9)