libbacktrace

This implements SOCK_DESTROY for UDP sockets similar to what was done
for TCP with commit c1e64e298b8ca ("net: diag: Support destroying TCP
sockets.") A process with a UDP socket targeted for destroy is awakened
and recvmsg fails with ECONNABORTED.

[backport of net-next 5d77dca82839ef016a93ad7acd7058b14d967752]

Change-Id: I84e71e774c859002f98dcdb5e0ca01f35227a44c
Signed-off-by: David Ahern <dsa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>