tracing: Add a seq_buf_clear() helper and clear len and readpos in init

Add a helper function seq_buf_clear() that resets the len and readpos
fields of a seq_buf. Currently it is only used in the seq_buf_init()
but will be used later when updating the seq_file code.

Link: http://lkml.kernel.org/r/20141104160222.352309995@goodmis.org

Tested-by: Jiri Kosina <jkosina@suse.cz>
Acked-by: Jiri Kosina <jkosina@suse.cz>
Reviewed-by: Petr Mladek <pmladek@suse.cz>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>