A root mount namespace list is used to resolve paths to
unix sockets if they are placed on btrfs.

This patch fixes a crash:
 #0 mount_resolve_path at criu/mount.c:213
 #1 phys_stat_resolve_dev at criu/mount.c:240
 #2 phys_stat_dev_match at criu/mount.c:256
 #3 unix_process_name at criu/sk-unix.c:565
 #4 unix_collect_one at criu/sk-unix.c:620
 #5 unix_receive_one at criu/sk-unix.c:692
 #6 nlmsg_receive at criu/libnetlink.c:45
 #7 do_rtnl_req at criu/libnetlink.c:119
 #8 do_collect_req at criu/sockets.c:610
 #9 collect_sockets at criu/sockets.c:636

travis-ci: success for cr-check: fill up a root task mount namespace
https://bugzilla.redhat.com/show_bug.cgi?id=1381351Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>