mountd: Add lookup_export_parent()

mountd: Add lookup_export_parent()

In a moment I will be adding some logic that needs to know an
junction's parent export.

Here's a function that can discover an export's parent.  It takes
the target export's pathname, chops off the rightmost component, and
tries a lookup_export().  If that succeeds, we have our answer.
If not, it chops off the next rightmost component and tries again,
until the root is reached.

At the same time, infrastructure is added to pass the parent export
down into the functions that convert locations into a new junction
export entry.  For now the parent export remains unused.

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