xfs: refactor and reuse best extent scanning logic

The bnobt "find best" helper implements a simple btree walker
function. This general pattern, or a subset thereof, is reused in
various parts of a near mode allocation operation. For example, the
bnobt left/right scans are each iterative btree walks along with the
cntbt lastblock scan.

Rework this function into a generic btree walker, add a couple
parameters to control termination behavior from various contexts and
reuse it where applicable.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>