dhcp: treat lease expiry as failure (rh #1139326)

Lease expiry means that the DHCP configuration is no longer valid, and
that all attempts to renew/rebind the lease have failed.  The IP config
needs to be removed.  NetworkManager also sets prefered/valid lifetimes
on addresses, so the kernel will remove them when the lease expires
anyway.  That causes removal of the default route, if the default route
was through the device whose config has now expired.

DHCP clients will typically move to the 'renew' or 'rebind' states when
nearing lease expiry, then if no answer is received move to the 'expire'
state.  Eventually they move to the 'fail' state when all attempts to
contact the server have failed.

Previously, since NM ignored the 'expire' DHCP state it would not clear
out the DHCP IP4 config immediately when the lease expired, instead
waiting for the DHCP client to move to the 'fail' state.  But if the
DHCP server appeared between the 'expire' and 'fail' states, NM would
not notice and the device's NMIP4Config would not change, and thus the
Policy would not get the "ip4-config-changed" signal to re-add the
default route that the kernel had previously removed due to the valid
lifetime reaching zero when the lease expired.

https://bugzilla.redhat.com/show_bug.cgi?id=1139326

src/dhcp-manager/nm-dhcp-client.c

@@ -178,7 +178,9 @@ reason_to_state (const char *iface, const char *reason)
 		return NM_DHCP_STATE_DONE;
 	else if (g_ascii_strcasecmp (reason, "fail") == 0 ||
 	         g_ascii_strcasecmp (reason, "abend") == 0 ||
-	         g_ascii_strcasecmp (reason, "nak") == 0)
+	         g_ascii_strcasecmp (reason, "nak") == 0 ||
+	         g_ascii_strcasecmp (reason, "expire") == 0 ||
+	         g_ascii_strcasecmp (reason, "expire6") == 0)
 		return NM_DHCP_STATE_FAIL;
 
 	nm_log_dbg (LOGD_DHCP, "(%s): unmapped DHCP state '%s'", iface, reason);