drm/fbdev-dma: Only set smem_start is enable per module option

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

drm/fbdev-dma: Only set smem_start is enable per module option

Only export struct fb_info.fix.smem_start if that is required by the

user and the memory does not come from vmalloc().

Setting struct fb_info.fix.smem_start breaks systems where DMA

memory is backed by vmalloc address space. An example error is

shown below.

[ 3.536043] ------------[ cut here ]------------

[ 3.540716] virt_to_phys used for non-linear address: 000000007fc4f540 (0xffff800086001000)

[ 3.552628] WARNING: CPU: 4 PID: 61 at arch/arm64/mm/physaddr.c:12 __virt_to_phys+0x68/0x98

[ 3.565455] Modules linked in:

[ 3.568525] CPU: 4 PID: 61 Comm: kworker/u12:5 Not tainted 6.6.23-06226-g4986cc3e1b75-dirty #250

[ 3.577310] Hardware name: NXP i.MX95 19X19 board (DT)

[ 3.582452] Workqueue: events_unbound deferred_probe_work_func

[ 3.588291] pstate: 60400009 (nZCv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)

[ 3.595233] pc : __virt_to_phys+0x68/0x98

[ 3.599246] lr : __virt_to_phys+0x68/0x98

[ 3.603276] sp : ffff800083603990

[ 3.677939] Call trace:

[ 3.680393] __virt_to_phys+0x68/0x98

[ 3.684067] drm_fbdev_dma_helper_fb_probe+0x138/0x238

[ 3.689214] __drm_fb_helper_initial_config_and_unlock+0x2b0/0x4c0

[ 3.695385] drm_fb_helper_initial_config+0x4c/0x68

[ 3.700264] drm_fbdev_dma_client_hotplug+0x8c/0xe0

[ 3.705161] drm_client_register+0x60/0xb0

[ 3.709269] drm_fbdev_dma_setup+0x94/0x148

Additionally, DMA memory is assumed to by contiguous in physical

address space, which is not guaranteed by vmalloc().

Resolve this by checking the module flag drm_leak_fbdev_smem when

DRM allocated the instance of struct fb_info. Fbdev-dma then only

sets smem_start only if required (via FBINFO_HIDE_SMEM_START). Also

guarantee that the framebuffer is not located in vmalloc address

space.