xfs: mark the record passed into xchk_btree functions as const

xchk_btree calls a user-supplied function to validate each btree record
that it finds.  Those functions are not supposed to change the record
data, so mark the parameter const.

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