pidfd: move struct pidfd_fops

Move the pidfd file operations over to their own file in preparation of
implementing pidfs and to isolate them from other mostly unrelated
functionality in other files.

Link: https://lore.kernel.org/r/20240213-vfs-pidfd_fs-v1-1-f863f58cfce1@kernel.org


Signed-off-by: Christian Brauner <brauner@kernel.org>