net/netvsc: control use of external mbuf on Rx

net/netvsc: control use of external mbuf on Rx

When receiving packets, netvsp puts data in a buffer mapped through UIO.
Depending on packet size, netvsc may attach the buffer as an external
mbuf. This is not a problem if this mbuf is consumed in the application,
and the application can correctly read data out of an external mbuf.

However, there are two problems with data in an external mbuf.
1. Due to the limitation of the kernel UIO implementation, physical
   address of this external buffer is not exposed to the user-mode. If
   this mbuf is passed to another driver, the other driver is unable to
   map this buffer to iova.
2. Some DPDK applications are not aware of external mbuf, and may bug
   when they receive an mbuf with external buffer attached.

Introduce a driver parameter "rx_extmbuf_enable" to control if netvsc
should use external mbuf for receiving packets. The default value is 0.
(netvsc doesn't use external mbuf, it always allocates mbuf and copy
data to mbuf) A non-zero value tells netvsc to attach external buffers
to mbuf on receiving packets, thus avoid copying memory.

Signed-off-by: Long Li <longli@microsoft.com>