scripts/ceph: Make sure osd_class_dir is an absolute path

Some Ceph builds, as seen under fedora37, have their default
osd_class_dir parameter set improperly to a relative path. As a
result, Ceph is not able to open and load .so libs that are part
of rados-classes. Example of an error seen in the ceph logs:

could not stat class lib64/rados-classes/libcls_rbd.so: (2) No such file or directory

To workaround it, always slap "/" at the beginning of the path.

Signed-off-by: Michal Berger <michal.berger@intel.com>
Change-Id: I08c2c94f7469748489ef4a9f0e65f3faccbecfb7
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/16064


Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Kamil Godzwon <kamilx.godzwon@intel.com>
Reviewed-by: Tomasz Zawadzki <tomasz.zawadzki@intel.com>
Reviewed-by: Changpeng Liu <changpeng.liu@intel.com>