s390/disassembler: correct disassembly lines alignment

176.718956 Krnl Code: 00000000004d38b0: a54c0018        llihh   %r4,24
176.718956 	   00000000004d38b4: b9080014        agr     %r1,%r4
           ^
Using a tab to align disassembly lines which follow the first line with
"Krnl Code: " doesn't always work, e.g. if there is a prefix (timestamp
or syslog prefix) which is not 8 chars aligned. Go back to alignment
with spaces.

Fixes: b192571d ("s390/disassembler: increase show_code buffer size")
Signed-off-by: Vasily Gorbik <gor@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>