net: Revert recent Spectre-v1 patches.

This reverts:

50d52586 ("net: core: Fix Spectre v1 vulnerability")
d686026b ("phonet: af_phonet: Fix Spectre v1 vulnerability")
a95386f0 ("nfc: af_nfc: Fix Spectre v1 vulnerability")
a3ac5817 ("can: af_can: Fix Spectre v1 vulnerability")

After some discussion with Alexei Starovoitov these all seem to
be completely unnecessary.

Signed-off-by: David S. Miller <davem@davemloft.net>