iomap: warn on zero-length mappings

Don't let the iomap callback get away with feeding us a garbage zero
length mapping -- there was a bug in xfs that resulted in those leaking
out to hilarious effect.

Signed-off-by: Darrick J. Wong <[e-mail]>
Reviewed-by: Christoph Hellwig <[e-mail]>

fs/iomap.c

···
 		return ret;
 	if (WARN_ON(iomap.offset > pos))
 		return -EIO;
+	if (WARN_ON(iomap.length == 0))
+		return -EIO;
 
 	/*
 	 * Cut down the length to the one actually provided by the filesystem,