RDMA/core: Use offsetofend() instead of open coding

No reason to open code this.

Link: https://lore.kernel.org/r/0-v1-0bc346e08476+585-drop_offsetofend_jgg@mellanox.com
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>

include/rdma/uverbs_ioctl.h

@@ -491,8 +491,7 @@ struct uapi_definition {
  */
 #define UVERBS_ATTR_STRUCT(_type, _last)                                       \
 	.zero_trailing = 1,                                                    \
-	UVERBS_ATTR_SIZE(((uintptr_t)(&((_type *)0)->_last + 1)),              \
-			 sizeof(_type))
+	UVERBS_ATTR_SIZE(offsetofend(_type, _last), sizeof(_type))
 /*
  * Specifies at least min_len bytes must be passed in, but the amount can be
  * larger, up to the protocol maximum size. No check for zeroing is done.