net: deprecate eth_change_mtu, remove usage

With centralized MTU checking, there's nothing productive done by
eth_change_mtu that isn't already done in dev_set_mtu, so mark it as
deprecated and remove all usage of it in the kernel. All callers have been
audited for calls to alloc_etherdev* or ether_setup directly, which means
they all have a valid dev->min_mtu and dev->max_mtu. Now eth_change_mtu
prints out a netdev_warn about being deprecated, for the benefit of
out-of-tree drivers that might be utilizing it.

Of note, dvb_net.c actually had dev->mtu = 4096, while using
eth_change_mtu, meaning that if you ever tried changing it's mtu, you
couldn't set it above 1500 anymore. It's now getting dev->max_mtu also set
to 4096 to remedy that.

v2: fix up lantiq_etop, missed breakage due to drive not compiling on x86

CC: netdev@vger.kernel.org
Signed-off-by: Jarod Wilson <jarod@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/uapi/linux/if_ether.h

@@ -35,6 +35,8 @@
 #define ETH_FRAME_LEN	1514		/* Max. octets in frame sans FCS */
 #define ETH_FCS_LEN	4		/* Octets in the FCS		 */
 
+#define ETH_MIN_MTU	68		/* Min IPv4 MTU per RFC791	*/
+
 /*
  *	These are the defined Ethernet Protocol ID's.
  */