xfs: make the key parameters to all btree key comparison functions const

xfs: make the key parameters to all btree key comparison functions const

Source kernel commit: d29d5577774d7d032da1343dba80be7423e307f9

The btree key comparison functions are not allowed to change the keys
that are passed in, so mark them const.  We'll need this for the next
patch, which adds const to the btree range query functions.

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