change find_ext_ns_id() to do collect_mntinfo(ext_ns) on demand
change find_ext_ns_id() to do collect_mntinfo(ext_ns) on demand

Currently we rely on the fact that ->mntinfo_list was already
collected by walk_namespaces(walk_all => true), but we are going
to change this.

This patch simply adds collect_mntinfo(ns) into find_ext_ns_id() if
->mntinfo_list == NULL. This is all we need for this ns_id if it was
not initialized by collect_mnt_namespaces().

Signed-off-by: Oleg Nesterov <20b70f0af00562e63758b9ee42012ecc96c58590@redhat.com>
Tested-by: Tycho Andersen <10c18fcb898ee6b17d6575d444b125d10b3f4d6a@canonical.com>
Signed-off-by: Pavel Emelyanov <c9a32589e048e044184536f7ac71ef92fe82df3e@parallels.com>

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