s390/zcrypt: Switch to flexible array member

s390/zcrypt: Switch to flexible array member

In preparation for FORTIFY_SOURCE performing compile-time and run-time
field bounds checking for memcpy(), memmove(), and memset(), avoid
intentionally reading/writing across neighboring array fields.

Switch from zero-element arrays to flexible arrays so the compiler will
perform appropriate bounds checking, avoiding these future warnings:

In function '__fortify_memcpy',
    inlined from 'cca_sec2protkey' at drivers/s390/crypto/zcrypt_ccamisc.c:645:2:
./include/linux/fortify-string.h:244:4: error: call to '__write_overflow_field' declared with attribute error: detected write beyond size of field (1st parameter)

In function '__fortify_memcpy',
    inlined from 'cca_clr2seckey' at drivers/s390/crypto/zcrypt_ccamisc.c:568:3:
./include/linux/fortify-string.h:246:4: error: call to '__read_overflow2_field' declared with attribute error: detected read beyond size of field (2nd parameter)

In function '__fortify_memcpy',
    inlined from 'cca_genseckey' at drivers/s390/crypto/zcrypt_ccamisc.c:429:2:
./include/linux/fortify-string.h:246:4: error: call to '__read_overflow2_field' declared with attribute error: detected read beyond size of field (2nd parameter)

Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20210616201903.1245949-1-keescook@chromium.org
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>