xfs: create a function to query all records in a btree

xfs: create a function to query all records in a btree

Source kernel commit: e9a2599a249ed7d31771985aea0e761f5680de64

Create a helper function that will query all records in a btree.
This will be used by the online repair functions to examine every
record in a btree to rebuild a second btree.

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