xfs: fix rmap inefficiencies Reduce the performance impact of the reverse mapping btree when reflink is enabled by using the much faster non-overlapped btree lookup functions when we're searching the rmap index with a fully specified key. If we find the exact record we're looking for, great! We don't have to perform the full overlapped scan. For filesystems with high sharing factors this reduces the xfs_scrub runtime by a good 15%%. This has been shown to reduce the fstests runtime for realtime rmap configurations by 30%%, since the lack of AGs severely limits scalability. v2: simplify the non-overlapped lookup code per dave comments