selftests/ftrace: Fix required features for VFS type test case

Since the VFS type argument test case uses fprobe events, it must
check the availablity of dynamic_events file and fprobe events syntax
in README. Without this fix, the test fails if CONFIG_FPROBE_EVENTS=n.

Link: https://lore.kernel.org/all/171478301645.110267.464634740467398506.stgit@devnote2/

Fixes: ee97e5e1

 ("selftests/ftrace: add fprobe test cases for VFS type "%pd" and "%pD"")
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>