s390/uaccess: Separate key uaccess functions

s390/uaccess: Separate key uaccess functions

Implement separate raw_copy_to_user_key() and raw_copy_from_user_key()
functions, which allows to remove the open-coded operand access control
handling from the normal raw_copy_to_user() / raw_copy_from_user()
functions - they are simplified to use immediate instructions to load
hard-coded operand access control values into register zero, which saves
one instruction.

Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>