dm btree: fix for dm_btree_find_lowest_key()

BugLink: http://bugs.launchpad.net/bugs/1694621

commit 7d1fedb6 upstream.

dm_btree_find_lowest_key() is giving incorrect results.  find_key()
traverses the btree correctly for finding the highest key, but there is
an error in the way it traverses the btree for retrieving the lowest
key.  dm_btree_find_lowest_key() fetches the first key of the rightmost
block of the btree instead of fetching the first key from the leftmost
block.

Fix this by conditionally passing the correct parameter to value64()
based on the @find_highest flag.
Signed-off-by: Erez Zadok <ezk@fsl.cs.sunysb.edu>
Signed-off-by: Vinothkumar Raja <vinraja@cs.stonybrook.edu>
Signed-off-by: Nidhi Panpalia <npanpalia@cs.stonybrook.edu>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>

drivers/md/persistent-data/dm-btree.c

@@ -887,8 +887,12 @@ static int find_key(struct ro_spine *s, dm_block_t block, bool find_highest,
 		else
 			*result_key = le64_to_cpu(ro_node(s)->keys[0]);
 
-		if (next_block || flags & INTERNAL_NODE)
-			block = value64(ro_node(s), i);
+		if (next_block || flags & INTERNAL_NODE) {
+			if (find_highest)
+				block = value64(ro_node(s), i);
+			else
+				block = value64(ro_node(s), 0);
+		}
 
 	} while (flags & INTERNAL_NODE);