uml: correctly handle skb allocation failures

Handle memory allocation failures when reading packets.

We have to read something from the host, even if we can't allocate any
memory.  If we don't, the host side of the device may fill up and stop
delivering interrupts because no new packets can be queued.

A single sk_buff is allocated whenever an MTU is seen which is larger
than any seen earlier.  This is used to read packets if there is a
memory allocation failure.

The large MTU check is done from eth_configure, which is called when a
interface is added to the system.

Signed-off-by: Jeff Dike <jdike@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>