powerpc/64: Fix checksum folding in csum_tcpudp_nofold and ip_fast_csum_nofold

These functions compute an IP checksum by computing a 64-bit sum and
folding it to 32 bits (the "nofold" in their names refers to folding
down to 16 bits).  However, doing (u32) (s + (s >> 32)) is not
sufficient to fold a 64-bit sum to 32 bits correctly.  The addition
can produce a carry out from bit 31, which needs to be added in to
the sum to produce the correct result.

To fix this, we copy the from64to32() function from lib/checksum.c
and use that.

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>