s390/crypto: Fix kernel crash on aes_s390 module remove.

s390/crypto: Fix kernel crash on aes_s390 module remove.

A kernel crash occurs when the aes_s390 kernel module is
removed on machines < z14. This only happens on kernel
version 4.15 and higher on machines not supporting MSA 8.

The reason for the crash is a unconditional
crypto_unregister_aead() invocation where no previous
crypto_register_aead() had been called. The fix now
remembers if there has been a successful registration and
only then calls the unregister function upon kernel module
remove.

The code now crashing has been introduced with
"bf7fa03 s390/crypto: add s390 platform specific aes gcm support."

Signed-off-by: Harald Freudenberger <freude@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>