docs: locking: Drop :c:func: throughout

The kernel doc tooling knows how to do this itself so drop this markup
throughout this file to simplify.
Suggested-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Stephen Boyd <swboyd@chromium.org>
Link: https://lore.kernel.org/r/20200318174133.160206-3-swboyd@chromium.orgSigned-off-by: Jonathan Corbet <corbet@lwn.net>