krbd: return error when no initial monitor address found

krbd: return error when no initial monitor address found

Since we filter monitor addresses based on ms_mode, check that at
least one address was found.

Otherwise, we mismatch arguments when calling sysfs/add_single_major
which emits a misleading error message to dmesg:

  libceph: resolve 'name=user1' (ret=-3): failed
  libceph: parse_ips bad ip 'name=user1,key=client.user1'

Fixes: https://tracker.ceph.com/issues/54128
Signed-off-by: Burt Holzman <burt@fnal.gov>