ocfs2: Rename 'user_stack' plugin structure to 'ocfs2_user_plugin'

The static structure describing the userspace cluster plugin for ocfs2
was named 'user_stack', which is a real pain when people are grep(1)ing
the tree for the program stack object 'user_stack'.  Change the name to
something distinct and namespaced.

Signed-off-by: Joel Becker <joel.becker@oracle.com>
Signed-off-by: Mark Fasheh <mfasheh@suse.com>