The bpf_csum_diff() helper has been fixed to return a 16-bit value for
all archs, so now we don't need to mask the result.
This commit is basically reverting the below:
commit 6185266c5a85 ("selftests/bpf: Mask bpf_csum_diff() return value
to 16 bits in test_verifier")
Signed-off-by: Puranjay Mohan <puranjay@xxxxxxxxxx>