riscv: refactor __get_user and __put_user

Add new __get_user_nocheck and __put_user_nocheck that switch on the size
and call the actual inline assembly helpers, and move the uaccess enable
/ disable into the actual __get_user and __put_user.  This prepares for
natively implementing __get_kernel_nofault and __put_kernel_nofault.

Also don't bother with the deprecated register keyword for the error
return.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>