staging: hv: fix netvsc sleeping while atomic

commit d70c673153d42e8aefd5ac296c8159ef222d076b upstream.

The channel callbacks are called directly from vmbus_event_dpc
which runs in tasklet context. These callbacks need to use
GFP_ATOMIC.

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

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