s390/zcore: fix race when reading from hardware system area

s390/zcore: fix race when reading from hardware system area

Memory buffer used for reading out data from hardware system
area is not protected against concurrent access.

Reported-by: Matthew Wilcox <willy@infradead.org>
Fixes: 5db25acea036 ("[S390] zfcpdump support.")
Acked-by: Heiko Carstens <hca@linux.ibm.com>
Tested-by: Alexander Egorenkov <egorenar@linux.ibm.com>
Link: https://lore.kernel.org/r/e68137f0f9a0d2558f37becc20af18e2939934f6.1658206891.git.agordeev@linux.ibm.com
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>