drm/gud: Fix UBSAN warning

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

drm/gud: Fix UBSAN warning

UBSAN complains about invalid value for bool:

[ 101.165172] [drm] Initialized gud 1.0.0 20200422 for 2-3.2:1.0 on minor 1

[ 101.213360] gud 2-3.2:1.0: [drm] fb1: guddrmfb frame buffer device

[ 101.213426] usbcore: registered new interface driver gud

[ 101.989431] ================================================================================

[ 101.989441] UBSAN: invalid-load in linux/include/linux/iosys-map.h:253:9

[ 101.989447] load of value 121 is not a valid value for type '_Bool'

[ 101.989451] CPU: 1 PID: 455 Comm: kworker/1:6 Not tainted 5.18.0-rc5-gud-5.18-rc5 #3

[ 101.989456] Hardware name: Hewlett-Packard HP EliteBook 820 G1/1991, BIOS L71 Ver. 01.44 04/12/2018

[ 101.989459] Workqueue: events_long gud_flush_work [gud]

[ 101.989471] Call Trace:

[ 101.989474] <TASK>

[ 101.989479] dump_stack_lvl+0x49/0x5f

[ 101.989488] dump_stack+0x10/0x12

[ 101.989493] ubsan_epilogue+0x9/0x3b

[ 101.989498] __ubsan_handle_load_invalid_value.cold+0x44/0x49

[ 101.989504] dma_buf_vmap.cold+0x38/0x3d

[ 101.989511] ? find_busiest_group+0x48/0x300

[ 101.989520] drm_gem_shmem_vmap+0x76/0x1b0 [drm_shmem_helper]

[ 101.989528] drm_gem_shmem_object_vmap+0x9/0xb [drm_shmem_helper]

[ 101.989535] drm_gem_vmap+0x26/0x60 [drm]

[ 101.989594] drm_gem_fb_vmap+0x47/0x150 [drm_kms_helper]

[ 101.989630] gud_prep_flush+0xc1/0x710 [gud]

[ 101.989639] ? _raw_spin_lock+0x17/0x40

[ 101.989648] gud_flush_work+0x1e0/0x430 [gud]

[ 101.989653] ? __switch_to+0x11d/0x470

[ 101.989664] process_one_work+0x21f/0x3f0

[ 101.989673] worker_thread+0x200/0x3e0

[ 101.989679] ? rescuer_thread+0x390/0x390

[ 101.989684] kthread+0xfd/0x130

[ 101.989690] ? kthread_complete_and_exit+0x20/0x20

[ 101.989696] ret_from_fork+0x22/0x30

[ 101.989706] </TASK>

[ 101.989708] ================================================================================

The source of this warning is in iosys_map_clear() called from

dma_buf_vmap(). It conditionally sets values based on map->is_iomem. The

iosys_map variables are allocated uninitialized on the stack leading to

->is_iomem having all kinds of values and not only 0/1.

Fix this by zeroing the iosys_map variables.