ftrace: Decrement count for dyn_ftrace_total_info file

The dyn_ftrace_total_info file is used to show how many functions have been
converted into nops and can be used by ftrace. The problem is that it does
not get decremented when functions are removed (init boot code being freed,
and modules being freed). That means the number is very inaccurate everytime
functions are removed from the ftrace tables. Decrement it when functions
are removed.

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