MINOR: buffer: split bi_contig_data() into ci_contig_data and b_config_data()

This function was sometimes used from a channel and sometimes from a buffer.
In both cases it requires knowledge of the size of the output data (to skip
them). Here the split ensures the channel can deal with this point, and that
other places not having output data can continue to work.
diff --git a/include/proto/channel.h b/include/proto/channel.h
index 9429689..9568050 100644
--- a/include/proto/channel.h
+++ b/include/proto/channel.h
@@ -309,6 +309,12 @@
 }
 
 
+/* Returns the amount of input data that can contiguously be read at once */
+static inline size_t ci_contig_data(const struct channel *c)
+{
+	return b_contig_data(c->buf, co_data(c));
+}
+
 /* Initialize all fields in the channel. */
 static inline void channel_init(struct channel *chn)
 {