ftrace/x86: Get the current mcount addr for add_breakpoint()

The add_breakpoint() code in the ftrace updating gets the address
of what the call will become, but if the mcount address is changing
from regs to non-regs ftrace_caller or vice versa, it will use what
the record currently is.

This is rather silly as the code should always use what is currently
there regardless of if it's changing the regs function or just converting
to a nop.

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