ocfs2: Increment the reference count of an already-active stack.

The ocfs2_stack_driver_request() function failed to increment the
refcount of an already-active stack.  It only did the increment on the
first reference.  Whoops.

Signed-off-by: Joel Becker <joel.becker@oracle.com>
Tested-by: Marcos Matsunaga <marcos.matsunaga@oracle.com>
Signed-off-by: Mark Fasheh <mfasheh@suse.com>