s390: make use of user_mode() macro where possible

We use the user_mode() helper already at several places but also
have the open coded variant at other places.
Convert the code to always use the helper function.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>