frr-reload: rpki context exiting uses exit and not end

frr-reload: rpki context exiting uses exit and not end

Issue:
The rpki subcontext uses exit instead of end to exit.
This makes issues with frr-reload in the way that frr-reload never exits
rpki context until it reaches the next end statement. this also happens when
parsing the configuration from vtysh.

Fixes: #7887
Signed-off-by: Runar Borge <runar@borge.nu>