Phonet: allocate sock from accept syscall rather than soft IRQ

This moves most of the accept logic to process context like other
socket stacks do. Then we can use a few more common socket helpers
and simplify a bit.

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