vfs: fix isize/pos/len checks for reflink & dedupe

Strengthen the checking of pos/len vs. i_size, clarify the return values
for the clone prep function, and remove pointless code.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/ocfs2/refcounttree.c

@@ -4834,7 +4834,7 @@ int ocfs2_reflink_remap_range(struct file *file_in,
 
 	ret = vfs_clone_file_prep_inodes(inode_in, pos_in, inode_out, pos_out,
 			&len, is_dedupe);
-	if (ret || len == 0)
+	if (ret <= 0)
 		goto out_unlock;
 
 	/* Lock out changes to the allocation maps and remap. */