RDMA/rxe: Extend rxe user ABI to support flush

This commit extends the rxe user ABI to support the flush
operation defined in IBA A19.4.1. These changes are
backward compatible with the existing rxe user ABI.

The user API request a flush by filling this structure.

Link: https://lore.kernel.org/r/[e-mail]
Reviewed-by: Zhu Yanjun <[e-mail]>
Signed-off-by: Li Zhijian <[e-mail]>
Signed-off-by: Jason Gunthorpe <[e-mail]>

include/uapi/rdma/rdma_user_rxe.h

···
 	union {
 		struct {
 			__aligned_u64 remote_addr;
+			__u32	length;
+			__u32	rkey;
+			__u8	type;
+			__u8	level;
+		} flush;
+		struct {
+			__aligned_u64 remote_addr;
 			__u32	rkey;
 			__u32	reserved;
 		} rdma;