ftrace: Add freeing algorithm to free ftrace_mod_maps

The ftrace_mod_map is a descriptor to save module init function names in
case they were traced, and the trace output needs to reference the function
name from the function address. But after the function is unloaded, it
the maps should be freed, as the rest of the function names are as well.

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