Btrfs: send, avoid incorrect leaf accesses when sending utimes operations

The caller of send_utimes() is supposed to be sure that the inode number
it passes to this function does actually exists in the send snapshot.
However due to logic/algorithm bugs (such as the one fixed by the patch
titled "Btrfs: send, fix invalid leaf accesses due to incorrect utimes
operations"), this might not be the case and when that happens it makes
send_utimes() access use an unrelated leaf item as the target inode item
or access beyond a leaf's boundaries (when the leaf is full and
path->slots[0] matches the number of items in the leaf).

So if the call to btrfs_search_slot() done by send_utimes() does not find
the inode item, just make sure send_utimes() returns -ENOENT and does not
silently accesses unrelated leaf items or does invalid leaf accesses, also
allowing us to easialy and deterministically catch such algorithmic/logic
bugs.
Signed-off-by: Filipe Manana <fdmanana@suse.com>

fs/btrfs/send.c

@@ -2502,6 +2502,8 @@ verbose_printk("btrfs: send_utimes %llu\n", ino);
 	key.type = BTRFS_INODE_ITEM_KEY;
 	key.offset = 0;
 	ret = btrfs_search_slot(NULL, sctx->send_root, &key, path, 0, 0);
+	if (ret > 0)
+		ret = -ENOENT;
 	if (ret < 0)
 		goto out;