alpha: fix alignment problem in csum_ipv6_magic()

Hopefully this fixes http://bugzilla.kernel.org/show_bug.cgi?id=8635

The struct in6_addr passed to csum_ipv6_magic() is 4 byte aligned, so we
can't use the regular 64-bit loads.  Since the cost of handling of 4 byte
and 1 byte aligned 64-bit data is roughly the same, this code can cope with
any src/dst [mis]alignment.

Signed-off-by: Ivan Kokshaysky <ink@jurassic.park.msu.ru>
Cc: Richard Henderson <rth@twiddle.net>
Cc: Dustin Marquess <jailbird@alcatraz.fdf.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>