ibmvnic fix NULL tx_pools and rx_tools issue at do_reset

At the time of do_rest, ibmvnic tries to re-initalize the tx_pools
and rx_pools to avoid re-allocating the long term buffer. However
there is a window inside do_reset that the tx_pools and
rx_pools were freed before re-initialized making it possible to deference
null pointers.

This patch fix this issue by always check the tx_pool
and rx_pool are not NULL after ibmvnic_login. If so, re-allocating
the pools. This will avoid getting into calling reset_tx/rx_pools with
NULL adapter tx_pools/rx_pools pointer. Also add null pointer check in
reset_tx_pools and reset_rx_pools to safe handle NULL pointer case.

Signed-off-by: Mingming Cao <mmc@linux.vnet.ibm.com>
Signed-off-by: Dany Madden <drt@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>