set IgnoreOnIsolate=true for systemd-cryptsetup@.service

When crypttab contains noauto, cryptsetup service does not have any
explicit dependencies. If service is started later manually (directly or via
mount dependency) it will be stopped on isolate.

mount units already have IgnoreOnIsolate set by default. Set it by
default for cryptsetup units as well.

src/cryptsetup/cryptsetup-generator.c

@@ -111,6 +111,7 @@ static int create_disk(
                 "Conflicts=umount.target\n"
                 "DefaultDependencies=no\n"
                 "BindsTo=dev-mapper-%i.device\n"
+                "IgnoreOnIsolate=true\n"
                 "After=systemd-readahead-collect.service systemd-readahead-replay.service\n",
                 f);