sparc32: propagate the calling conventions change down to __csum_partial_copy_sparc_generic()

... and get rid of zeroing the target, etc. on fault.
All exception handlers merge into one; moreover, since we are not
calling lookup_fault() anymore, we don't need the magic with passing
arguments for it from the page fault handler.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>