mountd: Dynamically allocate exportent that represents junctions

mountd: Dynamically allocate exportent that represents junctions

We're now duplicating a real exportent with arbitrary export options
to create a junction exportent.  After a dupexportent() call,
several of the structure's fields can point to dynamically allocated
memory.  We have to be careful about not orphaning that memory.

What's more, returning a pointer to a static structure is as 90's as
a bad mullet.  It's more straightforward to allocate the exportent
dynamically and release it when we are through with it.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Steve Dickson <steved@redhat.com>