arm64: Fix swiotlb fallback allocation

Commit b67a8b29 introduced logic to skip swiotlb allocation when all memory
is DMA accessible anyway.

While this is a great idea, __dma_alloc still calls swiotlb code unconditionally
to allocate memory when there is no CMA memory available. The swiotlb code is
called to ensure that we at least try get_free_pages().

Without initialization, swiotlb allocation code tries to access io_tlb_list
which is NULL. That results in a stack trace like this:

  Unable to handle kernel NULL pointer dereference at virtual address 00000000
  [...]
  [<ffff00000845b908>] swiotlb_tbl_map_single+0xd0/0x2b0
  [<ffff00000845be94>] swiotlb_alloc_coherent+0x10c/0x198
  [<ffff000008099dc0>] __dma_alloc+0x68/0x1a8
  [<ffff000000a1b410>] drm_gem_cma_create+0x98/0x108 [drm]
  [<ffff000000abcaac>] drm_fbdev_cma_create_with_funcs+0xbc/0x368 [drm_kms_helper]
  [<ffff000000abcd84>] drm_fbdev_cma_create+0x2c/0x40 [drm_kms_helper]
  [<ffff000000abc040>] drm_fb_helper_initial_config+0x238/0x410 [drm_kms_helper]
  [<ffff000000abce88>] drm_fbdev_cma_init_with_funcs+0x98/0x160 [drm_kms_helper]
  [<ffff000000abcf90>] drm_fbdev_cma_init+0x40/0x58 [drm_kms_helper]
  [<ffff000000b47980>] vc4_kms_load+0x90/0xf0 [vc4]
  [<ffff000000b46a94>] vc4_drm_bind+0xec/0x168 [vc4]
  [...]

Thankfully swiotlb code just learned how to not do allocations with the FORCE_NO
option. This patch configures the swiotlb code to use that if we decide not to
initialize the swiotlb framework.

Fixes: b67a8b29 ("arm64: mm: only initialize swiotlb when necessary")
Signed-off-by: Alexander Graf <agraf@suse.de>
CC: Jisheng Zhang <jszhang@marvell.com>
CC: Geert Uytterhoeven <geert+renesas@glider.be>
CC: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/mm/init.c

@@ -404,6 +404,8 @@ void __init mem_init(void)
 	if (swiotlb_force == SWIOTLB_FORCE ||
 	    max_pfn > (arm64_dma_phys_limit >> PAGE_SHIFT))
 		swiotlb_init(1);
+	else
+		swiotlb_force = SWIOTLB_NO_FORCE;
 
 	set_max_mapnr(pfn_to_page(max_pfn) - mem_map);