xfrm: set ipv4 no_pmtu_disc flag only on output sa when direction is set

The XFRM_STATE_NOPMTUDISC flag is only meaningful for output SAs, but
it was being applied regardless of the SA direction when the sysctl
ip_no_pmtu_disc is enabled. This can unintentionally affect input SAs.

Limit setting XFRM_STATE_NOPMTUDISC to output SAs when the SA direction
is configured.

Closes: https://github.com/strongswan/strongswan/issues/2946
Fixes: a4a87fa4e96c ("xfrm: Add Direction to the SA in or out")
Signed-off-by: Antony Antony <[e-mail]>
Signed-off-by: Steffen Klassert <[e-mail]>

net/xfrm/xfrm_state.c

···
 	int err;
 
 	if (family == AF_INET &&
+	    (!x->dir || x->dir == XFRM_SA_DIR_OUT) &&
 	    READ_ONCE(xs_net(x)->ipv4.sysctl_ip_no_pmtu_disc))
 		x->props.flags |= XFRM_STATE_NOPMTUDISC;