xfs: move comment about two 2 keys per pointer in the rmap btree

Move it to the relevant initialization of the ops structure instead
of a place that has nothing to do with the key size.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>