netlink: Always copy on mmap TX.

[ Upstream commit 4682a0358639b29cf69437ed909c6221f8c89847 ]

Checking the file f_count and the nlk->mapped count is not completely
sufficient to prevent the mmap'd area contents from changing from
under us during netlink mmap sendmsg() operations.

Be careful to sample the header's length field only once, because this
could change from under us as well.

Fixes: 5fd96123 ("netlink: implement memory mapped sendmsg()")
Signed-off-by: David S. Miller <davem@davemloft.net>
Acked-by: Daniel Borkmann <dborkman@redhat.com>
Acked-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>