powerpc/ftrace: Use generic ftrace_modify_all_code()

Convert powerpc's arch_ftrace_update_code() from its own version to use
the generic default functionality (without stop_machine -- our
instructions are properly aligned and the replacements atomic).

With this we gain error checking and the much-needed function_trace_op
handling.

Reviewed-by: Balbir Singh <bsingharora@gmail.com>
Reviewed-by: Kamalesh Babulal <kamalesh@linux.vnet.ibm.com>
Signed-off-by: Torsten Duwe <duwe@suse.de>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>