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 <[email protected]>
Signed-off-by: Stephen Boyd <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jonathan Corbet <[email protected]>

Author: bebarino