packet: fix races in fanout_add()

Multiple threads can call fanout_add() at the same time.

We need to grab fanout_mutex earlier to avoid races that could
lead to one thread freeing po->rollover that was set by another thread.

Do the same in fanout_release(), for peace of mind, and to help us
finding lockdep issues earlier.

Change-Id: Ie773328bb8b39ad04d73e35b6f8ec10716a8d9a1
Fixes: dc99f600 ("packet: Add fanout support.")
Fixes: 0648ab70afe6 ("packet: rollover prepare: per-socket state")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Git-commit: d199fab63c11998a602205f7ee7ff7c05c97164b
Git-repo: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


[tejaswit@codeaurora.org: rollover support not present in branch,
                          these parts removed from patch. ]
Signed-off-by: Tejaswi Tanikella <tejaswit@codeaurora.org>