Updated to remove a division by zero

Using btrfs as fs lead to a report of 0 inodes, creating a division by zero:

[2014-04-18 20:58:26,570] [Thread-11] Traceback (most recent call last):
  File "/usr/lib/pymodules/python2.7/diamond/collector.py", line 423, in _run
    self.collect()
  File "/usr/share/diamond/collectors/diskspace/diskspace.py", line 263, in collect
    float(inodes_free) / float(inodes_total) * 100)
ZeroDivisionError: float division by zero

Test : 

>>> data = os.statvfs('/opt/data')
>>> data
posix.statvfs_result(f_bsize=4096, f_frsize=4096, f_blocks=44564480, f_bfree=26219965, f_bavail=25761775, f_files=0, f_ffree=0, f_favail=0, f_flag=4096, f_namemax=255)


Will look into it to add btrfs support...