bcache: Add make_btree_freeing_key()
bcache: Add make_btree_freeing_key()

Refactoring, prep work for incremental garbage collection.

Signed-off-by: Kent Overstreet <99dffa8a9668ab7f6cde1c94b42a210109971113@daterainc.com>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs