It is best practice to use <portlet:namespace/> tag before the form name. This tag generates unique namespace for every portlet, so that naming conflicts won't occur even multiple portlets in a page have same form name.
Below code will be sollution to your problem. <portlet:actionURL > will invoke processAction() method of same portlet. So you can write your required logic in the processAction() method. Using render you can redirect to required page or jsp.
<form action="<portlet:actionURL ><portlet:param name="action" value="editStocks"/></portlet:actionURL>" method="POST" name ="<portlet:namespace/>fm">
You can also send parameters using <portlet:param> tag.