add SPDX tags to all block layer files

[Christoph Hellwig]

Hi Jens,

this series adds SPDX tags to all block layer files that are still
missing them.  The last patch adds them to files that didn't have
any licensing, and I've cced everyone who is mentioned in the
Copyright notices for these files to make sure no one has any
disagreement with the fact that that they are per default under
the kernels GPLv2 license.