s390/entry: make oklabel within CHKSTG macro local

Make the oklabel within the CHKSTG macro local. This makes sure that
tools like objdump and the crash debugging tool still disassemble full
functions where the macro has been used instead of stopping half way
where such a global label is used and one has to guess how to
disassemble the rest of such a function:

E.g.:

0000000000cb0270 <mcck_int_handler>:
  cb0270:       b2 05 03 20             stck    800
  ...
  cb0354:       a7 74 00 97             jne     cb0482 <oklabel270+0xe2>

0000000000cb0358 <oklabel243>:
  cb0358:       c0 e0 00 22 4e 8f       larl    %r14,10fa076 <opcode+0x2558>
  ...

Fixes: d35925b3 ("s390/mcck: move storage error checks to assembler")
Reviewed-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>