Re: Add a SOCK_DESTROY operation to close sockets from userspace

[Lorenzo Colitti <lorenzo@google.com>]

Here is a an updated version. The external behaviour of this
patchset is the same as v4; for more details, see that cover
letter at http://www.spinics.net/lists/netdev/msg354303.html .

This version fixes two bugs spotted by Eric, and implements Tom's
suggestion of making the socket destroy code a per-protocol
function pointer so that in-kernel callers can use it. The
resulting code is a bit longer but a bit more generic, and
exposes fewer TCP implementation details.

The operation is still called SOCK_DESTROY, but given that its
main implementation is the TCP ABORT operation, and that the
word "destroy" is used in the inet_csk code to refer to freeing
a socket, and in the inet_diag code to refer to broadcasts about
sockets being freed, perhaps it could be renamed to SOCK_ABORT.

Tested using net_test. Tests check that TCP resets are sent in
the right states, that accept(), read() and connect() are
interrupted, that closing sockets makes the socket unusable,
and that destroying non-TCP sockets returns EOPNOTSUPP. Tests
at https://android-review.googlesource.com/#/c/187491/ .