It fixes memory leak in ring buffer change logic.

When ring buffer size is changed(ethtool -G eth0 rx 4096), sfc driver
works like below.
1. stop all channels and remove ring buffers.
2. allocates new buffer array.
3. allocates rx buffers.
4. start channels.

While the above steps are working, it skips some steps if the channel
doesn't have a ->copy callback function.
Due to ptp channel doesn't have ->copy callback, these above steps are
skipped for ptp channel.
It eventually makes some problems.
a. ptp channel's ring buffer size is not changed, it works only
   1024(default).
b. memory leak.

The reason for memory leak is to use the wrong ring buffer values.
There are some values, which is related to ring buffer size.
a. efx->rxq_entries
 - This is global value of rx queue size.
b. rx_queue->ptr_mask
 - used for access ring buffer as circular ring.
 - roundup_pow_of_two(efx->rxq_entries) - 1
c. rx_queue->max_fill
 - efx->rxq_entries - EFX_RXD_HEAD_ROOM

These all values should be based on ring buffer size consistently.
But ptp channel's values are not.
a. efx->rxq_entries
 - This is global(for sfc) value, always new ring buffer size.
b. rx_queue->ptr_mask
 - This is always 1023(default).
c. rx_queue->max_fill
 - This is new ring buffer size - EFX_RXD_HEAD_ROOM.

Let's assume we set 4096 for rx ring buffer,

                      normal channel     ptp channel
efx->rxq_entries      4096               4096
rx_queue->ptr_mask    4095               1023
rx_queue->max_fill    4086               4086

sfc driver allocates rx ring buffers based on these values.
When it allocates ptp channel's ring buffer, 4086 ring buffers are
allocated then, these buffers are attached to the allocated array.
But ptp channel's ring buffer array size is still 1024(default)
and ptr_mask is still 1023 too.
So, 3062 ring buffers will be overwritten to the array.
This is the reason for memory leak.

Test commands:
   ethtool -G <interface name> rx 4096
   while :
   do
       ip link set <interface name> up
       ip link set <interface name> down
   done

In order to avoid this problem, it adds ->copy callback to ptp channel
type.
So that rx_queue->ptr_mask value will be updated correctly.

Fixes: 7c236c43 ("sfc: Add support for IEEE-1588 PTP")
Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>