f_phonet: use page-sized rather than MTU-sized RX buffers

Instead of a large (physically) linear buffer, we generate a set of
paged sk_buff, so no extra memory copy is involved. This removes
high-order allocations and saves quite a bit of memory. Phonet MTU is
65541 bytes, so the two buffers were padded to 128 kilo-bytes each.
Now, we create 17 page buffers, almost a 75% memory use reduction.

Signed-off-by: Rémi Denis-Courmont <remi.denis-courmont@nokia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>