sparc: fix accurate exception reporting in copy_{from,to}_user for M7

sparc: fix accurate exception reporting in copy_{from,to}_user for M7

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. This will fix the return value of copy_from_user and
copy_to_user in the faulting case. The behaviour of memcpy stays unchanged.

Fixes: 34060b8fffa7 ("arch/sparc: Add accurate exception reporting in M7memcpy")
Tested-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de> # on Oracle SPARC S7
Tested-by: Tony Rodriguez <unixpro1970@gmail.com> # S7, see https://lore.kernel.org/r/98564e2e68df2dda0e00c67a75c7f7dfedb33c7e.camel@physik.fu-berlin.de
Signed-off-by: Michael Karcher <kernel@mkarcher.dialup.fu-berlin.de>
Reviewed-by: Andreas Larsson <andreas@gaisler.com>
Link: https://lore.kernel.org/r/20250905-memcpy_series-v4-5-1ca72dda195b@mkarcher.dialup.fu-berlin.de
Signed-off-by: Andreas Larsson <andreas@gaisler.com>