pidfs: use anon_inode_setattr()

So far pidfs did use it's own version. Just use the generic version.
We use our own wrappers because we're going to be implementing
properties soon.

Link: https://lore.kernel.org/20250407-work-anon_inode-v1-4-53a44c20d44e@kernel.org


Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>