xprtrdma: fix incorrect header size calculations

Currently the header size calculations are using an assignment
operator instead of a += operator when accumulating the header
size leading to incorrect sizes.  Fix this by using the correct
operator.

Addresses-Coverity: ("Unused value")
Fixes: 302d3deb ("xprtrdma: Prevent inline overflow")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>

net/sunrpc/xprtrdma/rpc_rdma.c

@@ -71,7 +71,7 @@ static unsigned int rpcrdma_max_call_header_size(unsigned int maxsegs)
 	size = RPCRDMA_HDRLEN_MIN;
 
 	/* Maximum Read list size */
-	size = maxsegs * rpcrdma_readchunk_maxsz * sizeof(__be32);
+	size += maxsegs * rpcrdma_readchunk_maxsz * sizeof(__be32);
 
 	/* Minimal Read chunk size */
 	size += sizeof(__be32);	/* segment count */
@@ -94,7 +94,7 @@ static unsigned int rpcrdma_max_reply_header_size(unsigned int maxsegs)
 	size = RPCRDMA_HDRLEN_MIN;
 
 	/* Maximum Write list size */
-	size = sizeof(__be32);		/* segment count */
+	size += sizeof(__be32);		/* segment count */
 	size += maxsegs * rpcrdma_segment_maxsz * sizeof(__be32);
 	size += sizeof(__be32);	/* list discriminator */