s390/ftrace: fix ftrace_caller/ftrace_regs_caller generation

ftrace_caller was used for both ftrace_caller and ftrace_regs_caller,
which means that the target address of the hotpatch trampoline was
never updated.

With commit 89497968 ("s390/ftrace: provide separate
ftrace_caller/ftrace_regs_caller implementations") a separate
ftrace_regs_caller entry point was implemeted, however it was
forgotten to implement the necessary changes for ftrace_modify_call
and ftrace_make_call, where the branch target has to be modified
accordingly.

Therefore add the missing code now.

Fixes: 89497968 ("s390/ftrace: provide separate ftrace_caller/ftrace_regs_caller implementations")
Reviewed-by: Sven Schnelle <svens@linux.ibm.com>
Acked-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>