common: convert to SPDX license tags

common: convert to SPDX license tags

These have been scripted conversions then cleaned up by hand as
there was no consistency to the formatting of the license headers in
the common/ directory. Author information was also removed (it's in
the git history) and so now the header format is consistently:

##/bin/bash
# SPDX-License-Identifier: GPL-2.0(+)
# Copyright (c) <date> <owner>. All Rights Reserved.
#
# <file description>

Signed-off-by: Dave Chinner <dchinner@redhat.com>