find-esp: return pinned fd to ESP/XBOOTLDR

find-esp: return pinned fd to ESP/XBOOTLDR

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
a98a6eb95cc980edab4b0f9c59e6573edc7ffe0c. Hence, let's clean this up,
and pin the inode we really care about and return it.