overflow: Fix -Wtype-limits compilation warnings

Attempt to use check_shl_overflow() with inputs of unsigned type
produces the following compilation warnings.

drivers/infiniband/hw/mlx5/qp.c: In function _set_user_rq_size_:
./include/linux/overflow.h:230:6: warning: comparison of unsigned
expression >= 0 is always true [-Wtype-limits]
   _s >= 0 && _s < 8 * sizeof(*d) ? _s : 0;  \
      ^~
drivers/infiniband/hw/mlx5/qp.c:5820:6: note: in expansion of macro _check_shl_overflow_
  if (check_shl_overflow(rwq->wqe_count, rwq->wqe_shift,
&rwq->buf_size))
      ^~~~~~~~~~~~~~~~~~
./include/linux/overflow.h:232:26: warning: comparison of unsigned expression < 0 is always false [-Wtype-limits]
  (_to_shift != _s || *_d < 0 || _a < 0 ||   \
                          ^
drivers/infiniband/hw/mlx5/qp.c:5820:6: note: in expansion of macro _check_shl_overflow_
  if (check_shl_overflow(rwq->wqe_count, rwq->wqe_shift, &rwq->buf_size))
      ^~~~~~~~~~~~~~~~~~
./include/linux/overflow.h:232:36: warning: comparison of unsigned expression < 0 is always false [-Wtype-limits]
  (_to_shift != _s || *_d < 0 || _a < 0 ||   \
                                    ^
drivers/infiniband/hw/mlx5/qp.c:5820:6: note: in expansion of macro _check_shl_overflow_
  if (check_shl_overflow(rwq->wqe_count, rwq->wqe_shift,&rwq->buf_size))
      ^~~~~~~~~~~~~~~~~~

Fixes: 0c668477 ("overflow.h: Add arithmetic shift helper")
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>

include/linux/overflow.h

@@ -36,6 +36,12 @@
 #define type_max(T) ((T)((__type_half_max(T) - 1) + __type_half_max(T)))
 #define type_min(T) ((T)((T)-type_max(T)-(T)1))
 
+/*
+ * Avoids triggering -Wtype-limits compilation warning,
+ * while using unsigned data types to check a < 0.
+ */
+#define is_non_negative(a) ((a) > 0 || (a) == 0)
+#define is_negative(a) (!(is_non_negative(a)))
 
 #ifdef COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW
 /*
@@ -227,9 +233,9 @@
 	typeof(d) _d = d;						\
 	u64 _a_full = _a;						\
 	unsigned int _to_shift =					\
-		_s >= 0 && _s < 8 * sizeof(*d) ? _s : 0;		\
+		is_non_negative(_s) && _s < 8 * sizeof(*d) ? _s : 0;	\
 	*_d = (_a_full << _to_shift);					\
-	(_to_shift != _s || *_d < 0 || _a < 0 ||			\
+	(_to_shift != _s || is_negative(*_d) || is_negative(_a) ||	\
 	(*_d >> _to_shift) != _a);					\
 })