relay: allow the use of const callback structs

None of the relay users require the use of mutable structs for callbacks,
however the relay code does.  Instead of assigning the default callback
for subbuf_start, add a wrapper to conditionally call the client callback
if available, and fall back to default behaviour otherwise.

This lets all relay users make their struct rchan_callbacks const data.

[[email protected]: cleanups, per Christoph]
  Link: https://lkml.kernel.org/r/[email protected]

Link: https://lkml.kernel.org/r/cc3ff292e4eb4fdc56bee3d690c7b8e39209cd37.1606153547.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>
Cc: Jens Axboe <[email protected]>
Cc: Kalle Valo <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

0 files changed, 0 insertions, 0 deletions