xfs: work around bogus gcc warning in xfs_allocbt_init_cursor

GCC 4.6 complains about an array subscript is above array bounds when
using the btree index to index into the agf_levels array.  The only
two indices passed in are 0 and 1, and we have an assert insuring that.

Replace the trick of using the array index directly with using constants
in the already existing branch for assigning the XFS_BTREE_LASTREC_UPDATE
flag.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Alex Elder <aelder@sgi.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>