sparc: fix accurate exception reporting in copy_{from_to}_user for Niagara

In the Linux kernel, the following vulnerability has been resolved: sparc: fix accurate exception reporting in copy_{from_to}_user for Niagara The referenced commit introduced exception handlers on user-space memory references in copy_from_user and copy_to_user. These handlers return from the respective function and calculate the remaining bytes left to copy using the current register contents. This commit fixes a couple of bad calculations and a broken epilogue in the exception handlers. This will prevent crashes and ensure correct return values of copy_from_user and copy_to_user in the faulting case. The behaviour of memcpy stays unchanged.