IB/rdmavt: Fix target union member for rvt_post_one_wr()

The "cplen" result used by the memcpy() into struct rvt_swqe "wqe" may
be sized to 80 for struct rvt_ud_wr (which is member "ud_wr", not "wr"
which is only 40 bytes in size). Change the destination union member so
the compiler can use the correct bounds check.

struct rvt_swqe {
        union {
                struct ib_send_wr wr;   /* don't use wr.sg_list */
                struct rvt_ud_wr ud_wr;
		...
	};
	...
};

Silences false positive memcpy() run-time warning:

  memcpy: detected field-spanning write (size 80) of single field "&wqe->wr" at drivers/infiniband/sw/rdmavt/qp.c:2043 (size 40)

Reported-by: Zhang Yi <yizhan@redhat.com>
Link: https://bugzilla.kernel.org/show_bug.cgi?id=216561
Cc: Dennis Dalessandro <dennis.dalessandro@cornelisnetworks.com>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: Leon Romanovsky <leon@kernel.org>
Cc: linux-rdma@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20230218185701.never.779-kees@kernel.org
Reviewed-by: Dennis Dalessandro <dennis.dalessandro@cornelisnetworks.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>