powerpc: ftrace, use create_branch

Impact: clean up

Paul Mackerras pointed out that the code to determine if the branch
can reach the destination is incorrect. Michael Ellerman suggested
to pull out the code from create_branch and use that.

Simply using create_branch is probably the best.

Reported-by: Michael Ellerman <michael@ellerman.id.au>
Reported-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>