sockfs: switch to ->free_inode()

we do have an RCU-delayed part there already (freeing the wq),
so it's not like the pipe situation; moreover, it might be
worth considering coallocating wq with the rest of struct sock_alloc.
->sk_wq in struct sock would remain a pointer as it is, but
the object it normally points to would be coallocated with
struct socket...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>