Drivers: hv: vmbus: vmbus_sendpacket_ctl: hvsock: avoid unnecessary signaling

Drivers: hv: vmbus: vmbus_sendpacket_ctl: hvsock: avoid unnecessary signaling

BugLink: http://bugs.launchpad.net/bugs/1541585
When the hvsock channel's outbound ringbuffer is full (i.e.,
hv_ringbuffer_write() returns -EAGAIN), we should avoid the unnecessary
signaling the host.

Signed-off-by: Dexuan Cui <decui@microsoft.com>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
(cherry picked from commit 5f363bc38f810d238d1e8b19998625ddec3b8138)
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>