staging: hv: convert vmbus_on_msg_dpc to not call osd_schedule_callback

The additional abstraction is unneeded. This also fixes a sleeping
while atomic issue as osd_schedule_callback can sleep which is
not allowed for vmbus_on_msg_dpc running in a tasklet.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=16701

Reviewed-By: Hank Janssen <hjanssen@microsoft.com>
Cc: Haiyang Zhang <haiyangz@microsoft.com>
Signed-off-by: Timo Teräs <timo.teras@iki.fi>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>