s390/pkey: Simplify protected key length calculation code

The calculation of the length of a protected key based on
the protected key type is scattered over certain places within
the pkey code. By introducing a new inline function
pkey_keytype_to_size() this can be centralized and the calling
code can be reduced and simplified.

With this also comes a slight rework of the generation of
protected keys. Now the pkey_pckmo module is able to generate
all but ECC keys.

Signed-off-by: Harald Freudenberger <freude@linux.ibm.com>
Reviewed-by: Holger Dengler <dengler@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>