iosys-map: Fix undefined behavior in iosys_map_clear()

The current iosys_map_clear() implementation reads the potentially
uninitialized 'is_iomem' boolean field to decide which union member
to clear. This causes undefined behavior when called on uninitialized
structures, as 'is_iomem' may contain garbage values like 0xFF.

UBSAN detects this as:
UBSAN: invalid-load in include/linux/iosys-map.h:267
load of value 255 is not a valid value for type '_Bool'

Fix by unconditionally clearing the entire structure with memset(),
eliminating the need to read uninitialized data and ensuring all
fields are set to known good values.

Closes: https://gitlab.freedesktop.org/drm/i915/kernel/-/issues/14639
Fixes: 01fd30da0474 ("dma-buf: Add struct dma-buf-map for storing struct dma_buf.vaddr_ptr")
Signed-off-by: Nitin Gote <[e-mail]>
Reviewed-by: Andi Shyti <[e-mail]>
Reviewed-by: Thomas Zimmermann <[e-mail]>
Signed-off-by: Thomas Zimmermann <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

include/linux/iosys-map.h

···
  */
 static inline void iosys_map_clear(struct iosys_map *map)
 {
-	if (map->is_iomem) {
-		map->vaddr_iomem = NULL;
-		map->is_iomem = false;
-	} else {
-		map->vaddr = NULL;
-	}
+	memset(map, 0, sizeof(*map));
 }
 
 /**