ftrace: Expose ftrace_hash_empty and ftrace_lookup_ip

It will be used when checking graph filter hashes later.

Link: http://lkml.kernel.org/r/20170120024447.26097-2-namhyung@kernel.org



Signed-off-by: Namhyung Kim <namhyung@kernel.org>
[ Moved ftrace_hash dec and functions outside of FUNCTION_GRAPH define ]
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>