net: sock_diag: Fix spectre v1 gadget in __sock_diag_cmd()

req->sdiag_family is a user-controlled value that's used as an array
index. Sanitize it after the bounds check to avoid speculative
out-of-bounds array access.

This also protects the sock_is_registered() call, so this removes the
sanitize call there.

Fixes: e978de7a ("net: socket: Fix potential spectre v1 gadget in sock_is_registered")
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: konrad.wilk@oracle.com
Cc: jamie.iles@oracle.com
Cc: liran.alon@oracle.com
Cc: stable@vger.kernel.org
Signed-off-by: Jeremy Cline <jcline@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>