xfs: remove the get*keys and update_keys btree ops pointers

xfs: remove the get*keys and update_keys btree ops pointers

Source kernel commit: 973b83194bf12f7e315aace57ae2096ff7b82360

These are internal btree functions; we don't need them to be
dispatched via function pointers.  Make them static again and
just check the overlapped flag to figure out what we need to
do.  The strategy behind this patch was suggested by Christoph.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Suggested-by: Christoph Hellwig <hch@infradead.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>