Consolidated find/parent scanning functions

An interesting observation about the find and parent scanning functions
is that at their core, they're both actually doing the same operation.
They search a metadata-pair during fetch for an entry uses the entry data
instead of the entry tag. This means we can combine these functions and
get a decent code savings.

It's a little bit trickier because pair ordering isn't guaranteed. But
to work around that we can simply search for both pair orderings. It's a
bit more expensive but may be worth the code savings. A fancier
implementation in the future can avoid the 2x lfs_parent scans.

Author: geky