udp: fix race between close() and udp_abort()

Kaustubh reported and diagnosed a panic in udp_lib_lookup().
The root cause is udp_abort() racing with close(). Both
racing functions acquire the socket lock, but udp{v6}_destroy_sock()
release it before performing destructive actions.

We can't easily extend the socket lock scope to avoid the race,
instead use the SOCK_DEAD flag to prevent udp_abort from doing
any action when the critical race happens.

Change-Id: I8ec3f73ed5f8324722b29450c37b4ae23a81c0f3
Diagnosed-and-tested-by: Kaustubh Pandey <kapandey@codeaurora.org>
Fixes: 5d77dca8 ("net: diag: support SOCK_DESTROY for UDP sockets")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Git-Commit: a8b897c7bcd47f4147d066e22cc01d1026d7640e
Git-repo: https://kernel.googlesource.com/pub/scm/linux/kernel/git/netdev/net.git


Signed-off-by: Kaustubh Pandey <kapandey@codeaurora.org>