The reworks the ESP/XBOOTLDR logic to pin the ESP/XBOOTLDR via an fd,
and return that as optional return parameter.
So far we only pinned the parent dir of the ESP/XBOOTLDR, which was
useful when verifying that ESP/XBOOTLDR is actually a mount point by
comparing mount ids. This however became obsolete with
a98a6eb95c. Hence, let's clean this up,
and pin the inode we really care about and return it.