socket: do a generic_file_splice_read when proto_ops has no splice_read

splice(2) fails with -EINVAL when called reading on a socket with no splice_read
set in its proto_ops (such as vsock sockets). Switch this to fallbacks to a
generic_file_splice_read instead.

Signed-off-by: Slavomir Kaslev <kaslevs@vmware.com>
Signed-off-by: David S. Miller <davem@davemloft.net>