mnt: Rename _collect_root into _get_root_fd

Nowadays this routine is mainly used for getting an
fd, rather than keeping one for future reference.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
Revert "mnt: Merge two calls to collect_mntinfo on early restore"

This reverts commit f55a0b912c2bb105513b5f6e08a024958968ab9a.

The collected mounts list a) points to nsid and b) should be
put on nsid's mntinfo_tree. Thus is _should_ be done by separate
calls.