kunit: Fix again checksum tests on big endian CPUs

Commit b38460bc ("kunit: Fix checksum tests on big endian CPUs")
fixed endianness issues with kunit checksum tests, but then
commit 6f4c45cb

 ("kunit: Add tests for csum_ipv6_magic and
ip_fast_csum") introduced new issues on big endian CPUs. Those issues
are once again reflected by the warnings reported by sparse.

So, fix them with the same approach, perform proper conversion in
order to support both little and big endian CPUs. Once the conversions
are properly done and the right types used, the sparse warnings are
cleared as well.

Reported-by: Erhard Furtner <erhard_f@mailbox.org>
Fixes: 6f4c45cb

 ("kunit: Add tests for csum_ipv6_magic and ip_fast_csum")
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Tested-by: Charlie Jenkins <charlie@rivosinc.com>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Acked-by: Palmer Dabbelt <palmer@rivosinc.com>
Link: https://lore.kernel.org/r/73df3a9e95c2179119398ad1b4c84cdacbd8dfb6.1708684443.git.christophe.leroy@csgroup.eu
Signed-off-by: Jakub Kicinski <kuba@kernel.org>