mount: Reset mountinfos when mntns recreated

The list of mounts is used by inotify to restore a watch with open-by-handle. The problem
is that after the clean_mnt_ns is done the mntinfo list is no longer relevant.

Fix it by freeing the old one and initializing the new with the read-from-image stuff.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>