s390/zcrypt: Make apfs a real unsigned int field

s390/zcrypt: Make apfs a real unsigned int field

Slight rework on the apfs field: Instead of unsigned char[4]
make this a real 32 bit unsigned int field. With that done,
some assignments and some printouts can be simplified.
With that comes a slight move of the anonymous struct covering
the message type 86 header to dedupe some code lines.

Reviewed-by: Holger Dengler <dengler@linux.ibm.com>
Signed-off-by: Harald Freudenberger <freude@linux.ibm.com>
Acked-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>