csum_and_copy_from_user and csum_and_copy_to_user are exported by a few
architectures, but not actually used in modular code.  Drop the exports.

Link: https://lkml.kernel.org/r/20220421070440.1282704-1-hch@lst.deSigned-off-by: NChristoph Hellwig <hch@lst.de>
Acked-by: NJakub Kicinski <kuba@kernel.org>
Acked-by: NGeert Uytterhoeven <geert@linux-m68k.org>
Acked-by: NArnd Bergmann <arnd@arndb.de>
Acked-by: Michael Ellerman <mpe@ellerman.id.au> (powerpc)
Cc: David Miller <davem@davemloft.net>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>