ip-pool: Make host address valid even if prefix_len != 24

At the end of ip_pool_select_addr4() we'd check if the selected address
is equal to the subnet address and increment it by 1 to produce a valid
host address for the AP.  That check was always correct only with 24-bit
prefix, extend it to actually use the prefix-dependent mask instead of
0xff.  Fixes a testAP failure triggered 50% of the times because the
netmask is 28 bit long there.

src/ip-pool.c

@@ -237,7 +237,7 @@ check_avail:
 		selected -= count;
 	}
 
-	if ((host_addr & 0xff) == 0)
+	if ((host_addr & host_mask) == 0)
 		host_addr += 1;
 
 done: