s390/pkey: Add sysfs attributes to emit protected key blobs

Add binary read-only sysfs attributes for the pkey module
that can be used to read random protected keys from.
Keys are read from these attributes using a cat-like interface.

A typical use case for those keys is to encrypt a swap device
using the paes cipher. During processing of /etc/crypttab, the
random protected key to encrypt the swap device is read from
one of the attributes.

The following attributes are added:
  protkey/aes_128
  protkey/aes_192
  protkey/aes_256
  protkey/aes_128_xts
  protkey/aes_256_xts
Each attribute emits a protected key blob for the corresponding
key size and cipher mode.

Signed-off-by: Ingo Franzki <ifranzki@linux.ibm.com>
Reviewed-by: Harald Freudenberger <freude@linux.ibm.com>
Reviewed-by: Hendrik Brueckner <brueckner@linux.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>