Drivers: hv: vmbus: Eliminate the spin lock on the read path

Drivers: hv: vmbus: Eliminate the spin lock on the read path

BugLink: http://bugs.launchpad.net/bugs/1541585
The function hv_ringbuffer_read() is called always on a pre-assigned
CPU. Each chnnel is bound to a specific CPU and this function is
always called on the CPU the channel is bound. There is no need to
acquire the spin lock; get rid of this overhead.

Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
(cherry picked from commit 3eba9a77d5fc2cee486a16fff435686f024f61cf)
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>