zebra: Change zserv_accept to use global zsock FD

zebra: Change zserv_accept to use global zsock FD

Problem:
Zebra crashed while going down. This happened because zebra was
trying to process a new client accept request after closing ZAPI's
listener socket zsock and setting it to -1.

Fix:
Skip rescheduling zserv_accept() and accepting a new client if global
ZAPI listener socket FD, zsock is already closed and set to -1.
Also use global ZAPI listener socket FD zsock in zserv_accept() instead
of using a copy of zsock.

Signed-off-by: Pooja Jagadeesh Doijode <pdoijode@nvidia.com>