IB/qib: Use struct_size() helper

Make use of the struct_size() helper instead of an open-coded version
in order to avoid any potential type mistakes, in particular in the
context in which this code is being used.

So, replace the following form:

sizeof(*pkt) + sizeof(pkt->addr[0])*n

with:

struct_size(pkt, addr, n)

Also, notice that variable size is unnecessary, hence it is removed.

This code was detected with the help of Coccinelle.
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Reviewed-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>

drivers/infiniband/hw/qib/qib_user_sdma.c

@@ -904,10 +904,11 @@ static int qib_user_sdma_queue_pkts(const struct qib_devdata *dd,
 		}
 
 		if (frag_size) {
-			int pktsize, tidsmsize, n;
+			int tidsmsize, n;
+			size_t pktsize;
 
 			n = npages*((2*PAGE_SIZE/frag_size)+1);
-			pktsize = sizeof(*pkt) + sizeof(pkt->addr[0])*n;
+			pktsize = struct_size(pkt, addr, n);
 
 			/*
 			 * Determine if this is tid-sdma or just sdma.