ftrace: Add another check for match in register_ftrace_direct()

As an instruction pointer passed into register_ftrace_direct() may just
exist on the ftrace call site, but may not be the start of the call site
itself, register_ftrace_direct() still needs to update test if a direct call
exists on the normalized site, as only one direct call is allowed at any one
time.

Fixes: 763e34e7 ("ftrace: Add register_ftrace_direct()")
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>