ocfs2/dlm: use struct_size() helper

One of the more common cases of allocation size calculations is finding
the size of a structure that has a zero-sized array at the end, along
with memory for some number of elements for that array.  For example:

  struct dlm_migratable_lockres
  {
          ...
          struct dlm_migratable_lock ml[0];  // 16 bytes each, begins at byte 112
  };

Make use of the struct_size() helper instead of an open-coded version in
order to avoid any potential type mistakes.

So, replace the following form:

   sizeof(struct dlm_migratable_lockres) + (mres->num_locks * sizeof(struct dlm_migratable_lock))

with:

   struct_size(mres, ml, mres->num_locks)

Notice that, in this case, variable sz is not necessary, hence it is
removed.

This code was detected with the help of Coccinelle.

Link: http://lkml.kernel.org/r/20190605204926.GA24467@embeddedorSigned-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Reviewed-by: Joseph Qi <joseph.qi@linux.alibaba.com>
Cc: Mark Fasheh <mark@fasheh.com>
Cc: Joel Becker <jlbec@evilplan.org>
Cc: Junxiao Bi <junxiao.bi@oracle.com>
Cc: Changwei Ge <gechangwei@live.cn>
Cc: Gang He <ghe@suse.com>
Cc: Jun Piao <piaojun@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/ocfs2/dlm/dlmrecovery.c

@@ -1109,7 +1109,7 @@ static int dlm_send_mig_lockres_msg(struct dlm_ctxt *dlm,
 {
 	u64 mig_cookie = be64_to_cpu(mres->mig_cookie);
 	int mres_total_locks = be32_to_cpu(mres->total_locks);
-	int sz, ret = 0, status = 0;
+	int ret = 0, status = 0;
 	u8 orig_flags = mres->flags,
 	   orig_master = mres->master;
 
@@ -1117,9 +1117,6 @@ static int dlm_send_mig_lockres_msg(struct dlm_ctxt *dlm,
 	if (!mres->num_locks)
 		return 0;
 
-	sz = sizeof(struct dlm_migratable_lockres) +
-		(mres->num_locks * sizeof(struct dlm_migratable_lock));
-
 	/* add an all-done flag if we reached the last lock */
 	orig_flags = mres->flags;
 	BUG_ON(total_locks > mres_total_locks);
@@ -1133,7 +1130,8 @@ static int dlm_send_mig_lockres_msg(struct dlm_ctxt *dlm,
 
 	/* send it */
 	ret = o2net_send_message(DLM_MIG_LOCKRES_MSG, dlm->key, mres,
-				 sz, send_to, &status);
+				 struct_size(mres, ml, mres->num_locks),
+				 send_to, &status);
 	if (ret < 0) {
 		/* XXX: negative status is not handled.
 		 * this will end up killing this node. */