stmmac: align RX buffers

On RX an SKB is allocated and the received buffer is copied into it.
But on some architectures, the memcpy() needs the source and destination
buffers to have the same alignment to be efficient.

This is not our case, because SKB data pointer is misaligned by two bytes
to compensate the ethernet header.

Align the RX buffer the same way as the SKB one, so the copy is faster.
An iperf3 RX test gives a decent improvement on a RISC-V machine:

before:
[ ID] Interval           Transfer     Bitrate         Retr
[  5]   0.00-10.00  sec   733 MBytes   615 Mbits/sec   88             sender
[  5]   0.00-10.01  sec   730 MBytes   612 Mbits/sec                  receiver

after:
[ ID] Interval           Transfer     Bitrate         Retr
[  5]   0.00-10.00  sec  1.10 GBytes   942 Mbits/sec    0             sender
[  5]   0.00-10.00  sec  1.09 GBytes   940 Mbits/sec                  receiver

And the memcpy() overhead during the RX drops dramatically.

before:
Overhead  Shared O  Symbol
  43.35%  [kernel]  [k] memcpy
  33.77%  [kernel]  [k] __asm_copy_to_user
   3.64%  [kernel]  [k] sifive_l2_flush64_range

after:
Overhead  Shared O  Symbol
  45.40%  [kernel]  [k] __asm_copy_to_user
  28.09%  [kernel]  [k] memcpy
   4.27%  [kernel]  [k] sifive_l2_flush64_range

Signed-off-by: Matteo Croce <mcroce@microsoft.com>
Signed-off-by: David S. Miller <davem@davemloft.net>