Add extra arch overrides to asm-generic/checksum.h

There are plausible reasons for architectures to provide their own
versions of csum_partial_copy_nocheck and csum_tcpudp_magic.
By protecting these, the architecture can still re-use the
asm-generic checksum.h, instead of copying it.

Signed-off-by: Linas Vepstas <linas@codeaurora.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Richard Kuo <rkuo@codeaurora.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>