ibmvnic: Reuse rx pools when possible

Rather than releasing the rx pools on and reallocating them on every
reset, reuse the rx pools unless the pool parameters (number of pools,
size of each pool or size of each buffer in a pool) have changed.

If the pool parameters changed, then release the old pools (if any)
and allocate new ones.

Specifically release rx pools, if:
	- adapter is removed,
	- pool parameters change during reset,
	- we encounter an error when opening the adapter in response
	  to a user request (in ibmvnic_open()).

and don't release them:
	- in __ibmvnic_close() or
	- on errors in __ibmvnic_open()

in the hope that we can reuse them on the next reset.

With these, reset_rx_pools() can be dropped because its optimzation is
now included in init_rx_pools() itself.

cleanup_rx_pools() releases all the skbs associated with the pool and
is called from ibmvnic_cleanup(), which is called on every reset. Since
we want to reuse skbs across resets, move cleanup_rx_pools() out of
ibmvnic_cleanup() and call it only when user closes the adapter.

Add two new adapter fields, ->prev_rx_buf_sz, ->prev_rx_pool_size to
keep track of the previous values and use them to decide whether to
reuse or realloc the pools.

Reviewed-by: Rick Lindsley <ricklind@linux.vnet.ibm.com>
Reviewed-by: Dany Madden <drt@linux.ibm.com>
Signed-off-by: Sukadev Bhattiprolu <sukadev@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>