Staging: hv: netvsc: Fix a bug in accounting transmit slots

The transmit slots were manipulated without proper locking. Fix this bug by
making the variable tracking the transmit slots atomic.

This patch should be ported to prior stable kernels 2.6.32 and later.

Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Haiyang Zhang <haiyangz@microsoft.com>
Signed-off-by: Hank Janssen <hjanssen@microsoft.com>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>