s390/uaccess: get rid of not needed local variable

Get rid of the not needed val local variable and pass the constant
value directly as operand value. In addition this turns the val
operand into an input operand, since it is not changed within the
inline assemblies.

This in turn requires also to add the earlyclobber contraint modifier
to all output operands, since the (former) val operand is used after
all output variants have been modified.

The usercopy kunit tests still pass after this change.

Reviewed-by: Gerald Schaefer <gerald.schaefer@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>