xfrm: set err and extack on failure to create pcpu SA

xfrm_state_construct can fail without setting an error if the
requested pcpu_num value is too big. Set err and add an extack message
to avoid confusing userspace.

Fixes: 1ddf9916ac09 ("xfrm: Add support for per cpu xfrm state handling.")
Signed-off-by: Sabrina Dubroca <[e-mail]>
Signed-off-by: Steffen Klassert <[e-mail]>

net/xfrm/xfrm_user.c

···
 
 	if (attrs[XFRMA_SA_PCPU]) {
 		x->pcpu_num = nla_get_u32(attrs[XFRMA_SA_PCPU]);
-		if (x->pcpu_num >= num_possible_cpus())
+		if (x->pcpu_num >= num_possible_cpus()) {
+			err = -ERANGE;
+			NL_SET_ERR_MSG(extack, "pCPU number too big");
 			goto error;
+		}
 	}
 
 	err = __xfrm_init_state(x, extack);