drm/i915: properly sanity check batch_start_offset

Check the edge case where batch_start_offset sits exactly on the batch
size.

v2: add new range_overflows variant to capture the special case where
the size is permitted to be zero, like with batch_len.

v3: other way around. the common case is the exclusive one which should
just be >=, with that we then just need to convert the three odd ball
cases that don't apply to use the new inclusive _end version.

Testcase: igt/gem_exec_params/invalid-batch-start-offset
Fixes: 0b5372727b ("drm/i915/cmdparser: Use cached vmappings")
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20200306094735.258285-1-matthew.auld@intel.com

drivers/gpu/drm/i915/i915_utils.h

@@ -102,12 +102,24 @@ bool i915_error_injected(void);
 	typeof(max) max__ = (max); \
 	(void)(&start__ == &size__); \
 	(void)(&start__ == &max__); \
-	start__ > max__ || size__ > max__ - start__; \
+	start__ >= max__ || size__ > max__ - start__; \
 })
 
 #define range_overflows_t(type, start, size, max) \
 	range_overflows((type)(start), (type)(size), (type)(max))
 
+#define range_overflows_end(start, size, max) ({ \
+	typeof(start) start__ = (start); \
+	typeof(size) size__ = (size); \
+	typeof(max) max__ = (max); \
+	(void)(&start__ == &size__); \
+	(void)(&start__ == &max__); \
+	start__ > max__ || size__ > max__ - start__; \
+})
+
+#define range_overflows_end_t(type, start, size, max) \
+	range_overflows_end((type)(start), (type)(size), (type)(max))
+
 /* Note we don't consider signbits :| */
 #define overflows_type(x, T) \
 	(sizeof(x) > sizeof(T) && (x) >> BITS_PER_TYPE(T))