s390/ftrace: verify opcode before applying patch

commit 72b3942a ("scripts: ftrace - move the
sort-processing in ftrace_init") had the unexpected
side effect that wrong code locations were patched.
To prevent this from happening again, verify the
opcode before patching it.

Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>