drm/i915/selftests: Prevent selecting 0 for our random width/align

When igt_random_offset() is a given a range of [0, PAGE_SIZE], it is
allowed to return 0. However, attempting to use a size of 0 for the
igt_lmem_write_cpu() byte poking, leads to call igt_random_offset() with
a range of [offset, offset + 0] and ask it to find a length of 4 within
it. This triggers the bug on that the requested length should fit within
the range!
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: NMatthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200806145728.16495-1-chris@chris-wilson.co.ukSigned-off-by: NRodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: NJoonas Lahtinen <joonas.lahtinen@linux.intel.com>

drivers/gpu/drm/i915/selftests/intel_memory_region.c

@@ -522,9 +522,9 @@ static int igt_lmem_write_cpu(void *arg)
 		goto out_unpin;
 	}
 
-	/* We want to throw in a random width/align */
-	bytes[0] = igt_random_offset(&prng, 0, PAGE_SIZE, sizeof(u32),
-				     sizeof(u32));
+	/* A random multiple of u32, picked between [64, PAGE_SIZE - 64] */
+	bytes[0] = igt_random_offset(&prng, 64, PAGE_SIZE - 64, 0, sizeof(u32));
+	GEM_BUG_ON(!IS_ALIGNED(bytes[0], sizeof(u32)));
 
 	i = 0;
 	do {