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

Vulnerability Description

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.

References

• https://git.kernel.org/stable/c/05440320ea3e249d5f984918f2bf51210c1a7c03
• https://git.kernel.org/stable/c/7823fc4d8ab5e57f8db7806ff2530c03c166c4bb
• https://git.kernel.org/stable/c/37547d8e6eba87507279ee3dfddfd9dc46335454
• https://git.kernel.org/stable/c/a365ee556e45f780ee322b349a06efdad0c1458f
• https://git.kernel.org/stable/c/8cdeb5e482d3fdce7e825444b6ca3865e24c0228
• https://git.kernel.org/stable/c/a90ce516a73dbe087f9bf3dbf311301a58d125c6
• https://git.kernel.org/stable/c/088c5098ec6d6b0396edfbf3dad3e81de8469c1c
• https://git.kernel.org/stable/c/0b67c8fc10b13a9090340c5f8a37d308f4e1571c