seq_file/seq_buf: Convert seq_file over to use seq_buf

trace_seq.c had code that was very similar to seq_file but instead of
using a file handle, it was made to pass a buffer around that any
function could write a string to. Since this code duplicated seq_file
code, seq_buf.c was created to move the common code into its own space.
trace_seq.c already uses seq_buf, now it's time to have seq_file.c use
it too.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>