[SRU E/F] selftests: net: ip_defrag: fix spurious failures

[Thadeu Lima de Souza Cascardo]

BugLink: https://bugs.launchpad.net/bugs/1826848

One of those fixes has been accepted upstream. The other one is a workaround on
some limitation that was discussed upstream, and very hard to fix properly.

[Impact]
Failure to run ip_defrag deterministically.

[Fix]
Use smaller packets and ignore EPERM.

[Test case]
Run the test multiple times without observing failures.

[Regression potential]
If the test fix is incorrect, it will cause us to miss real kernel bugs.
But as it is now, we are already ignoring its results.