filelock: default to returning -EINVAL when ->setlease operation is NULL

filelock: default to returning -EINVAL when ->setlease operation is NULL

Now that most filesystems where we expect to need lease support have
their ->setlease() operations explicitly set, change kernel_setlease()
to return -EINVAL when the setlease is a NULL pointer.

Also update the Documentation/ with info about this change.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Link: https://patch.msgid.link/20260108-setlease-6-20-v1-23-ea4dec9b67fa@kernel.org
Acked-by: Al Viro <viro@zeniv.linux.org.uk>
Acked-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>