filelock: add some new helper functions

In later patches we're going to embed some common fields into a new
structure inside struct file_lock. Smooth the transition by adding some
new helper functions, and converting the core file locking code to use
them.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Link: https://lore.kernel.org/r/20240131-flsplit-v3-4-c6129007ee8d@kernel.org


Reviewed-by: NeilBrown <neilb@suse.de>
Signed-off-by: Christian Brauner <brauner@kernel.org>