rpm: Migrate license in ceph.spec to SPDX

rpm: Migrate license in ceph.spec to SPDX

All are straightforward but BSD. That should be either 2 or 3 clause. But at least arrow uses both variants.

Previous format is non-standard. In Fedora we call it Callaway, because
Tom Callway just made up the IDs.
Fedora is now moving to SPDX as an industry standard.
For more details see https://fedoraproject.org/wiki/Changes/SPDX_Licenses_Phase_4

Signed-off-by: Miroslav Suchý <msuchy@redhat.com>