net: netlink: Remove the dump_cb_mutex field from struct netlink_sock

Commit 5fbf57a9 ("net: netlink: remove the cb_mutex "injection" from
netlink core") has removed the usage of the 'dump_cb_mutex' field from the
struct netlink_sock.

Remove the field itself now. It saves a few bytes in the structure.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>