xfs: in btree_lshift, only allocate temporary cursor when needed

We only need the temporary cursor in _btree_lshift if we're shifting
in an overlapped btree.  Therefore, factor that into a single block
of code so we avoid unnecessary cursor duplication.

Also fix use of the wrong cursor when checking for corruption in
xfs_btree_rshift().

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>