virtio_net: migrate mergeable rx buffers to page frag allocators

The virtio_net driver's mergeable receive buffer allocator
uses 4KB packet buffers. For MTU-sized traffic, SKB truesize
is > 4KB but only ~1500 bytes of the buffer is used to store
packet data, reducing the effective TCP window size
substantially. This patch addresses the performance concerns
with mergeable receive buffers by allocating MTU-sized packet
buffers using page frag allocators. If more than MAX_SKB_FRAGS
buffers are needed, the SKB frag_list is used.

Signed-off-by: Michael Dalton <mwdalton@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>