btrfs: reada: Fix returned errno code

reada is using -1 instead of the -ENOMEM defined macro to specify that
a buffer allocation failed. Since the error number is propagated, the
caller will get a -EPERM which is the wrong error condition.

Also, updating the caller to return the exact value from
reada_add_block.

Smatch tool warning:
reada_add_block() warn: returning -1 instead of -ENOMEM is sloppy

Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: Luis de Bethencourt <luisbg@osg.samsung.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/reada.c

@@ -569,7 +569,7 @@ static int reada_add_block(struct reada_control *rc, u64 logical,
 	rec = kzalloc(sizeof(*rec), GFP_NOFS);
 	if (!rec) {
 		reada_extent_put(root->fs_info, re);
-		return -1;
+		return -ENOMEM;
 	}
 
 	rec->rc = rc;
@@ -918,6 +918,7 @@ struct reada_control *btrfs_reada_add(struct btrfs_root *root,
 	u64 start;
 	u64 generation;
 	int level;
+	int ret;
 	struct extent_buffer *node;
 	static struct btrfs_key max_key = {
 		.objectid = (u64)-1,
@@ -943,9 +944,10 @@ struct reada_control *btrfs_reada_add(struct btrfs_root *root,
 	generation = btrfs_header_generation(node);
 	free_extent_buffer(node);
 
-	if (reada_add_block(rc, start, &max_key, level, generation)) {
+	ret = reada_add_block(rc, start, &max_key, level, generation);
+	if (ret) {
 		kfree(rc);
-		return ERR_PTR(-ENOMEM);
+		return ERR_PTR(ret);
 	}
 
 	reada_start_machine(root->fs_info);