xfs: mark the record passed into btree init_key functions as const

xfs: mark the record passed into btree init_key functions as const

These functions initialize a key from a record, but they aren't supposed
to modify the record.  Mark it const.

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