[S390] kexec: Disable ftrace during kexec

Disable ftrace during kexec. Same as on x86/powerpc.
ac4414e4 "powerpc/kdump: Disable ftrace during kexec".

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>