vmxnet3: repair memory leak

with the introduction of commit
b0eb57cb

, it appears that rq->buf_info
is improperly handled.  While it is heap allocated when an rx queue is
setup, and freed when torn down, an old line of code in
vmxnet3_rq_destroy was not properly removed, leading to rq->buf_info[0]
being set to NULL prior to its being freed, causing a memory leak, which
eventually exhausts the system on repeated create/destroy operations
(for example, when  the mtu of a vmxnet3 interface is changed
frequently.

Fix is pretty straight forward, just move the NULL set to after the
free.

Tested by myself with successful results

Applies to net, and should likely be queued for stable, please

Signed-off-by: Neil Horman <nhorman@tuxdriver.com>
Reported-By:  <boyang@redhat.com>
CC: boyang@redhat.com
CC: Shrikrishna Khare <skhare@vmware.com>
CC: "VMware, Inc." <pv-drivers@vmware.com>
CC: David S. Miller <davem@davemloft.net>
Acked-by: Shrikrishna Khare <skhare@vmware.com>
Signed-off-by: David S. Miller <davem@davemloft.net>