Staging: hv: vmbus: Cleanup error values in ringbuffer.c

Use standard Linux errno values in ringbuffer.c and do the associated
cleanup in the clients.

Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Haiyang Zhang <haiyangz@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>