mountd: Use static buffer when constructing junction export options

Clean up: Simplify locations_to_export() by constructing a junction's
export options in a static buffer.

We can do this because all of this code is called serially, in one
thread, and the result is thrown away immediately after the caller
is finished.  The returned exportent itself is static.

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