[S390] uaccess: turn __access_ok() into a define

Turn __access_ok() into a define and add a __chk_user_ptr() call
instead.

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