f_phonet: fix skb truesize underestimation

Now skb_add_rx_frag() has a truesize parameter, we can fix f_phonet to
properly account truesize of each fragment : a full page.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Felipe Balbi <balbi@ti.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Rémi Denis-Courmont <remi.denis-courmont@nokia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>