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

Anthony Yznaga tracked down that a BUG_ON in ext4 code with large folios
enabled resulted from copy_from_user() returning impossibly large values
greater than the size to be copied. This lead to __copy_from_iter()
returning impossible values instead of the actual number of bytes it was
able to copy.

The BUG_ON has been reported in
https://lore.kernel.org/r/b14f55642207e63e907965e209f6323a0df6dcee.camel@physik.fu-berlin.de



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. The exception handlers expect that
%o2 has already been masked during the bulk copy loop, but the masking was
performed after that loop. 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: ee841d0a ("sparc64: Convert U3copy_{from,to}_user to accurate exception reporting.")
Tested-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de> # on Sun Netra 240
Reviewed-by: Anthony Yznaga <anthony.yznaga@oracle.com>
Tested-by: René Rebe <rene@exactcode.com> # on UltraSparc III+ and UltraSparc IIIi
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-v4-2-1ca72dda195b@mkarcher.dialup.fu-berlin.de


Signed-off-by: Andreas Larsson <andreas@gaisler.com>