rbd-nbd: map using netlink interface by default

rbd-nbd: map using netlink interface by default

Mapping rbd images to nbd devices using ioctl interface is not
robust. It was discovered that the device size or the md5 checksum
of the nbd device was incorrect immediately after mapping using
ioctl method. When using the nbd netlink interface to map RBD images
the issue was not encountered. Switch to using nbd netlink interface
for mapping.

Fixes: https://tracker.ceph.com/issues/64063
Signed-off-by: Ramana Raja <rraja@redhat.com>
(cherry picked from commit fcbf7367d285629b382e3d9d32ac354319d1cc66)

Conflicts:
	PendingReleaseNotes [ moved to >=18.2.5 section ]