relay: make create_buf_file and remove_buf_file callbacks mandatory

All clients provide create_buf_file and remove_buf_file callbacks, and
they're required for relay to make sense.  There is no point in them being
optional.

Also document whether each callback is mandatory/optional.

Link: https://lkml.kernel.org/r/88003c1527386b93036e286e7917f1e33aec84ac.1606153547.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Suggested-by: Christoph Hellwig <hch@infradead.org>
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>

include/linux/relay.h

@@ -89,6 +89,8 @@ struct rchan_callbacks
 	 * The client should return 1 to continue logging, 0 to stop
 	 * logging.
 	 *
+	 * This callback is optional.
+	 *
 	 * NOTE: subbuf_start will also be invoked when the buffer is
 	 *       created, so that the first sub-buffer can be initialized
 	 *       if necessary.  In this case, prev_subbuf will be NULL.
@@ -122,6 +124,8 @@ struct rchan_callbacks
 	 * cause relay_open() to create a single global buffer rather
 	 * than the default set of per-cpu buffers.
 	 *
+	 * This callback is mandatory.
+	 *
 	 * See Documentation/filesystems/relay.rst for more info.
 	 */
 	struct dentry *(*create_buf_file)(const char *filename,
@@ -139,6 +143,8 @@ struct rchan_callbacks
 	 * channel buffer.
 	 *
 	 * The callback should return 0 if successful, negative if not.
+	 *
+	 * This callback is mandatory.
 	 */
 	int (*remove_buf_file)(struct dentry *dentry);
 };