tracing: Add full state to trace_seq

The trace_seq buffer might fill up, and right now one needs to check the
return value of each printf into the buffer to check for that.

Instead, have the buffer keep track of whether it is full or not, and
reject more input if it is full or would have overflowed with an input
that wasn't added.

Cc: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

include/linux/trace_seq.h

@@ -14,6 +14,7 @@ struct trace_seq {
 	unsigned char		buffer[PAGE_SIZE];
 	unsigned int		len;
 	unsigned int		readpos;
+	int			full;
 };
 
 static inline void
@@ -21,6 +22,7 @@ trace_seq_init(struct trace_seq *s)
 {
 	s->len = 0;
 	s->readpos = 0;
+	s->full = 0;
 }
 
 /*