xfs: refactor near mode alloc bnobt scan into separate function

In preparation to enhance the near mode allocation bnobt scan algorithm, lift
it into a separate function. No functional changes.

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>