drm/amdgpu: Make SR-IOV critical region checks overflow-safe

drm/amdgpu: Make SR-IOV critical region checks overflow-safe

The function amdgpu_virt_init_critical_region() contained an invalid
check for a negative init_hdr_offset value:

    if (init_hdr_offset < 0)

Since init_hdr_offset is an unsigned 32-bit integer, this condition can
never be true and triggers a Smatch warning:

    warn: unsigned 'init_hdr_offset' is never less than zero

In addition, the subsequent bounds check: if ((init_hdr_offset +
init_hdr_size) > vram_size) was vulnerable to integer overflow when
adding the two unsigned values.  Thus, by promoting offset and size to
64-bit and using check_add_overflow() to safely validate the sum against
VRAM size.

Fixes: 07009df6494d ("drm/amdgpu: Introduce SRIOV critical regions v2 during VF init")
Reported by: Dan Carpenter <dan.carpenter@linaro.org>
Cc: Ellen Pan <yunru.pan@amd.com>
Cc: Christian König <christian.koenig@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
Reviewed-by: Ellen Pan <yunru.pan@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>