net: improve the user pointer check in init_user_sockptr

Make sure not just the pointer itself but the whole range lies in
the user address space.  For that pass the length and then use
the access_ok helper to do the check.

Fixes: 6d04fe15 ("net: optimize the sockptr_t for unified kernel/user address spaces")
Reported-by: David Laight <David.Laight@ACULAB.COM>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: David S. Miller <davem@davemloft.net>