IB/uverbs: Add a macro to define a type with no kernel known size

Sometimes the uverbs uAPI  doesn't really care about the structure it gets
from user-space. All it wants to do is to allocate enough space and send
it to the hardware/provider driver. Adding a UVERBS_ATTR_MIN_SIZE that
could be used for this scenarios. We use USHRT_MAX as the kernel known
size to bypass any zero validations.
Signed-off-by: Matan Barak <matanb@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>

drivers/infiniband/core/uverbs_std_types.c

@@ -207,10 +207,10 @@ int uverbs_destroy_def_handler(struct ib_device *ib_dev,
  * spec.
  */
 const struct uverbs_attr_def uverbs_uhw_compat_in =
-	UVERBS_ATTR_PTR_IN_SZ(UVERBS_ATTR_UHW_IN, UVERBS_ATTR_SIZE(0, USHRT_MAX),
+	UVERBS_ATTR_PTR_IN_SZ(UVERBS_ATTR_UHW_IN, UVERBS_ATTR_MIN_SIZE(0),
 			      UA_FLAGS(UVERBS_ATTR_SPEC_F_MIN_SZ_OR_ZERO));
 const struct uverbs_attr_def uverbs_uhw_compat_out =
-	UVERBS_ATTR_PTR_OUT_SZ(UVERBS_ATTR_UHW_OUT, UVERBS_ATTR_SIZE(0, USHRT_MAX),
+	UVERBS_ATTR_PTR_OUT_SZ(UVERBS_ATTR_UHW_OUT, UVERBS_ATTR_MIN_SIZE(0),
 			       UA_FLAGS(UVERBS_ATTR_SPEC_F_MIN_SZ_OR_ZERO));
 
 void create_udata(struct uverbs_attr_bundle *ctx, struct ib_udata *udata)

include/rdma/uverbs_ioctl.h

@@ -215,6 +215,8 @@ struct uverbs_object_tree_def {
 	.min_len = ((uintptr_t)(&((_type *)0)->_last + 1)), .len = sizeof(_type)
 #define UVERBS_ATTR_SIZE(_min_len, _len)			\
 	.min_len = _min_len, .len = _len
+#define UVERBS_ATTR_MIN_SIZE(_min_len)				\
+	UVERBS_ATTR_SIZE(_min_len, USHRT_MAX)
 
 /*
  * In new compiler, UVERBS_ATTR could be simplified by declaring it as