Now that relay_open() accepts const callbacks, make relay callbacks const.

Link: https://lkml.kernel.org/r/534d089f413db98aa0b94773fa49d5275d0d3c25.1606153547.git.jani.nikula@intel.comSigned-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Kalle Valo <kvalo@codeaurora.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>