docs: filesystems: convert locks.txt to ReST

- Add a SPDX header;
- Adjust document and section titles;
- Add it to filesystems/index.rst.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/467b3f20e63d2640d22599b99229699b5fb79251.1588021877.git.mchehab+huawei@kernel.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>