ibcore: Remove the use of NULL_IB_OBJECT

LinuxKPI's XArray implementation accepts NULL as an input as of the
following commit:

• linuxkpi: Accept NULL as a value in linux_xarray (3102ea3b15b6)

Sponsored by: Juniper Networks, Inc.
MFC after: 1 week
Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D44533