selftests/ftrace: Add user-memory access syntax testcase

Add a user-memory access syntax testcase which checks
new user-memory access syntax and ustring type.

Link: http://lkml.kernel.org/r/155789873385.26965.9557271156179140676.stgit@devnote2



Acked-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>