[S390] ftrace: drop nmi protection

The function graph tracer used to have a protection against NMI
while entering a function entry tracing. But this is useless now,
the tracer is reentrant and the ring buffer supports NMI tracing.

Same as 07868b08 for x86.

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