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.
-----BEGIN PGP SIGNATURE-----
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=4oW8
-----END PGP SIGNATURE-----