bgpd: Fix attribute handling for type-5 routes

bgpd: Fix attribute handling for type-5 routes

When a EVPN type-5 route is formed by using the source IP route's AS-path,
the AS-path is not locally generated and should not be "uninterned" (i.e.,
have its reference count incorrectly updated). An incorrect update of the
reference count can lead to asserts or crashes at a later stage.

Signed-off-by: Vivek Venkatraman <vivek@cumulusnetworks.com>
Ticket: CM-19121
Reviewed By: CCR-7028
Testing Done:
1. Manual testing by Vivek and Anitha
2. No automation run since this area has no coverage yet