misc: fastrpc: fix an incorrect NULL check on list iterator

The bug is here:
	if (!buf) {

The list iterator value 'buf' will *always* be set and non-NULL
by list_for_each_entry(), so it is incorrect to assume that the
iterator value will be NULL if the list is empty (in this case, the
check 'if (!buf) {' will always be false and never exit expectly).

To fix the bug, use a new variable 'iter' as the list iterator,
while use the original variable 'buf' as a dedicated pointer to
point to the found element.

Fixes: 2419e55e ("misc: fastrpc: add mmap/unmap support")
Signed-off-by: Xiaomeng Tong <xiam0nd.tong@gmail.com>
Link: https://lore.kernel.org/r/20220327062202.5720-1-xiam0nd.tong@gmail.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/misc/fastrpc.c

@@ -1606,17 +1606,18 @@ static int fastrpc_req_munmap_impl(struct fastrpc_user *fl,
 				   struct fastrpc_req_munmap *req)
 {
 	struct fastrpc_invoke_args args[1] = { [0] = { 0 } };
-	struct fastrpc_buf *buf, *b;
+	struct fastrpc_buf *buf = NULL, *iter, *b;
 	struct fastrpc_munmap_req_msg req_msg;
 	struct device *dev = fl->sctx->dev;
 	int err;
 	u32 sc;
 
 	spin_lock(&fl->lock);
-	list_for_each_entry_safe(buf, b, &fl->mmaps, node) {
-		if ((buf->raddr == req->vaddrout) && (buf->size == req->size))
+	list_for_each_entry_safe(iter, b, &fl->mmaps, node) {
+		if ((iter->raddr == req->vaddrout) && (iter->size == req->size)) {
+			buf = iter;
 			break;
-		buf = NULL;
+		}
 	}
 	spin_unlock(&fl->lock);