selftests/net: improve descriptions for XFAIL cases in psock_snd.sh

Before changing this it's a bit confusing to read test output:
  raw csum_off with bad offset (fails)
  ./psock_snd: write: Invalid argument

Change "fails" in the test case description to "expected to fail", so
that the test output can be more understandable.

Signed-off-by: Po-Hsu Lin <po-hsu.lin@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>