docs: filesystems: convert gfs2-glocks.txt to ReST

- Add a SPDX header;
- Adjust document and section titles;
- Some whitespace fixes and new line breaks;
- Add table markups;
- Use notes markups;
- Add it to filesystems/index.rst.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>