zebra: Limit mutex for obuf to when we access obuf

zebra: Limit mutex for obuf to when we access obuf

The mutex that wraps access to the output buffer
is being held for the entire time the data is
being generated to send down the pipe.  Since
the generation has absolutely nothing to do
with the obuf, let's limit the mutex holding some.

Signed-off-by: Donald Sharp <sharpd@nvidia.com>