landlock: Rename "ptrace" files to "task"

ptrace.[ch] are currently only used for the ptrace LSM hooks but their
scope will expand with IPCs and audit support. Rename ptrace.[ch] to
task.[ch], which better reflect their content. Similarly, rename
landlock_add_ptrace_hooks() to landlock_add_task_hooks(). Keep header
files for now.

Cc: Günther Noack <gnoack@google.com>
Cc: Paul Moore <paul@paul-moore.com>
Link: https://lore.kernel.org/r/20240307093923.1466071-2-mic@digikod.net
Signed-off-by: Mickaël Salaün <mic@digikod.net>