tunneling: Convert tunnel push/pop functions to act on single packets.

tunneling: Convert tunnel push/pop functions to act on single packets.

The userspace tunneling API for pushing and popping tunnel headers
is currently based on processing batches of packets. However, there
is no obvious way to take advantage of batching for these operations
and so each tunnel operation has a pair of loops to process the
batch. This changes the API to operate on single packets to enable
better code reuse.

Signed-off-by: Jesse Gross <jesse@nicira.com>
Acked-by: Pravin B Shelar <pshelar@nicira.com>