It's a common practice to start a search using offset (u64)-1, which is
the u64 maximum value, meaning that we want the search_slot function to
be set in the last item with the same objectid and type.

Once we are in this position, it's a matter to start a search backwards
by calling btrfs_previous_item, which will check if we'll need to go to
a previous leaf and other necessary checks, only to be sure that we are
in last offset of the same object and type. If the item is found,
convert the ondisk structure to the current endianness of the machine
running the code.

The new btrfs_search_backwards function does the all these procedures when
necessary, and can be used to avoid code duplication.

No functional changes.

Signed-off-by: Marcos Paulo de Souza <redacted>
---
 fs/btrfs/ctree.c   | 23 +++++++++++++++++++++++
 fs/btrfs/ctree.h   |  6 ++++++
 fs/btrfs/ioctl.c   | 30 ++++++++----------------------
 fs/btrfs/super.c   | 26 ++++++--------------------
 fs/btrfs/volumes.c |  7 +------
 5 files changed, 44 insertions(+), 48 deletions(-)

diff --git a/fs/btrfs/ctree.c b/fs/btrfs/ctree.c
index 394fec1d3fd9..2991ee845813 100644
--- a/fs/btrfs/ctree.c
+++ b/fs/btrfs/ctree.c

@@ -2100,6 +2100,29 @@ int btrfs_search_slot_for_read(struct btrfs_root *root,
 	return 0;
 }
 
+/*
+ * Execute search and call btrfs_previous_item to traverse backwards if the item
+ * was not found. If found, convert the stored item to the correct endianness.
+ *
+ * Return 0 if found, 1 if not found and < 0 if error.
+ */
+int btrfs_search_backwards(struct btrfs_root *root,
+				struct btrfs_key *key,
+				struct btrfs_key *found_key,