s390/uaccess: Remove __put_user_fn()/__get_user_fn() wrappers

The __put_user_fn() and __get_user_fn() wrappers are leftovers from the
time where the kernel was compiled with or without mvcos support plus they
were later used to workaround the problems that came with asm register
constructs.

Both reasons do not exist anymore, therefore remove the wrappers and
shorten the code.

Reviewed-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>