Metadata is kept per block to keep track of where the contig hints are.
Scanning can be avoided when the contig hints are not broken. In that
case, left and right contigs have to be managed manually.

This patch changes the allocation path hint updating to only scan when
contig hints are broken.
Signed-off-by: NDennis Zhou <dennisszhou@gmail.com>
Reviewed-by: NJosef Bacik <jbacik@fb.com>
Signed-off-by: NTejun Heo <tj@kernel.org>