Commit 658f2dc97 accidentally dropped the cast to the target type of
the value loaded by get_user(). The most visible effect of this would
be that the sequence "uint64_t v; get_user_u32(v, addr)" would sign
extend the 32 bit loaded value into v rather than zero extending as
would be expected for a _u32 accessor. Put the cast back again to
restore the old behaviour.