s390/uaccess: fix access_ok compile warnings

On s390 access_ok is a macro which discards all parameters and always
returns 1. This can result in compile warnings which warn about unused
variables like this:

fs/read_write.c: In function 'rw_copy_check_uvector':
fs/read_write.c:684:16: warning: unused variable 'buf' [-Wunused-variable]

Fix this by adding a __range_ok() function which consumes all parameters
but still always returns 1.

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