fs/iso9660: Add check to prevent infinite loop

fs/iso9660: Add check to prevent infinite loop

There is no check for the end of block when reading
directory extents. It resulted in read_node() always
read from the same offset in the while loop, thus
caused infinite loop. The fix added a check for the
end of the block and ensure the read is within directory
boundary.

Signed-off-by: Lidong Chen <lidong.chen@oracle.com>
Reviewed-by: Thomas Schmitt <scdbackup@gmx.net>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>