Phonet: fix aligned-mode pipe socket buffer header reserve

When the pipe uses aligned-mode data packets, we must reserve 4 bytes
instead of 3 for the pipe protocol header. Otherwise the Phonet header
would not be aligned, resulting in potentially corrupted headers with
later unaligned memory writes.

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