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

Link: https://lkml.kernel.org/r/534d089f413db98aa0b94773fa49d5275d0d3c25.1606153547.git.jani.nikula@intel.com


Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Kalle Valo <kvalo@codeaurora.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>