drm/amdgpu: Fix integer overflow in amdgpu_gem_add_input_fence()

The "num_syncobj_handles" is a u32 value that comes from the user via the
ioctl.  On 32bit systems the "sizeof(uint32_t) * num_syncobj_handles"
multiplication can have an integer overflow.  Use size_mul() to fix that.

Fixes: 38c67ec9 ("drm/amdgpu: Add input fence to sync bo map/unmap")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>