Fix uninitialized variable from r300881

sys/cddl/contrib/opensolaris/uts/common/fs/zfs/vdev_geom.c
Initialize needs_update in vdev_geom_set_physpath

PR: 210409
Reported by: kp
Reviewed by: kp
Approved by: re (hrs)
MFC after: 4 weeks
X-MFC-With: 300881
Sponsored by: Spectra Logic Corp