staging: erofs: update source file headers

- Use the correct style for all SPDX License Identifiers;
- Get rid of the unnecessary license boilerplate;
- Use "GPL-2.0-only" instead of "GPL-2.0" suggested-by Stephen.

Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
Link: https://lore.kernel.org/r/20190731155752.210602-2-gaoxiang25@huawei.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
20 files changed