tracing: Clean up tb_fmt to not give faulty compile warning

gcc incorrectly states that the variable "fmt" is uninitialized when
CC_OPITMIZE_FOR_SIZE is set.

Instead of just blindly setting fmt to NULL, the code is cleaned up
a little to be a bit easier for humans to follow, as well as gcc
to know the variables are initialized.

Change-Id: Ia5fb9cc6be8eadae9af3a5636a2459ed7cd48662
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Rohit Vaswani <rvaswani@codeaurora.org>